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collision and disruption processes of protoplanetary bodies in the early solar system are key to understanding the genesis of diverse types of main-belt asteroids. mesosiderites are stony-iron meteorites that formed by the mixing of howardite-eucrite-diogenite-like crust and molten core materials and provide unique insights into the catastrophic break-up of differentiated asteroids. however, the enigmatic formation process and the poorly constrained timing of metal-silicate mixing complicate the assignment to potential parent bodies. here we report the high-precision uranium-lead dating of mesosiderite zircons by isotope dilution thermal ionization mass spectrometry to reveal an initial crust formation 4,558.5 ± 2.1 million years ago and metal-silicate mixing at 4,525.39 ± 0.85 million years ago. the two distinct ages coincide with the timing of the crust formation and a large-scale reheating event on the eucrite parent body, probably the asteroid vesta. this chronological coincidence corroborates that vesta is the parent body of mesosiderite silicates. mesosiderite formation on vesta can be explained by a hit-and-run collision 4,525.4 million years ago that caused the thick crust observed by nasa's dawn mission and explains the missing olivine in mesosiderites, howardite-eucrite-diogenite meteorites and vestoids. | mesosiderite formation on asteroid 4 vesta by a hit-and-run collision |
the yarkovsky effect describes a small but significant force that affects the orbital motion of meteoroids and asteroids smaller than 30-40 km in diameter. it is caused by sunlight; when these bodies heat up in the sun, they eventually reradiate the energy away in the thermal waveband, which in turn creates a tiny thrust. this recoil acceleration is much weaker than solar and planetary gravitational forces, but it can produce measurable orbital changes over decades and substantial orbital effects over millions to billions of years. the same physical phenomenon also creates a thermal torque that, complemented by a torque produced by scattered sunlight, can modify the rotation rates and obliquities of small bodies as well. this rotational variant has been coined the yarkovsky-o'keefe-radzievskii-paddack (yorp) effect. during the past decade or so, the yarkovsky and yorp effects have been used to explore and potentially resolve a number of unsolved mysteries in planetary science dealing with small bodies. here we review the main results to date, and preview the goals for future work. | the yarkovsky and yorp effects |
the asteroid belt originated from leftover debris from the formation of our solar system, including objects that never grew big enough to become planets. destructive collisions led to the present proliferation of asteroids; groups that were initially part of the same body have related orbits and are known as families. delbo' et al. identified an ancient family mostly consisting of dark asteroids that previously had not been linked to families (see the perspective by demeo). from this, they calculated the original population of the belt, showing that it contained several dozen midsized bodies known as planetesimals. | identification of a primordial asteroid family constrains the original planetesimal population |
we studied a sample of 93 asteroid pairs, i.e., pairs of genetically related asteroids that are on highly similar heliocentric orbits. we estimated times elapsed since separation of pair members (i.e., pair age) that are between 7 × 103 yr and a few 106 yr. with photometric observations, we derived the rotation periods p1 for all the primaries (i.e., the larger members of asteroid pairs) and a sample of secondaries (the smaller pair members). we derived the absolute magnitude differences of the studied asteroid pairs that provide their mass ratios q. for a part of the studied pairs, we refined their wise geometric albedos and collected or estimated their taxonomic classifications. for 17 asteroid pairs, we also determined their pole positions. in two pairs where we obtained the spin poles for both pair components, we saw the same sense of rotation for both components and constrained the angles between their original spin vectors at the time of their separation. we found that the primaries of 13 asteroid pairs in our sample are actually binary or triple systems, i.e., they have one or two bound, orbiting secondaries (satellites). as a by-product, we found also 3 new young asteroid clusters (each of them consisting of three known asteroids on highly similar heliocentric orbits). we compared the obtained asteroid pair data with theoretical predictions and discussed their implications. we found that 86 of the 93 studied asteroid pairs follow the trend of primary rotation period vs mass ratio that was found by pravec et al. (2010). of the 7 outliers, 3 appear insignificant (may be due to our uncertain or incomplete knowledge of the three pairs), but 4 are high mass ratio pairs that were unpredicted by the theory of asteroid pair formation by rotational fission. we discuss a (remotely) possible way that they could be created by rotational fission of flattened parent bodies followed by re-shaping of the formed components. the 13 asteroid pairs with binary primaries are particularly interesting systems that place important constraints on formation and evolution of asteroid pairs. we present two hypotheses for their formation: the asteroid pairs having both bound and unbound secondaries could be "failed asteroid clusters", or they could be formed by a cascade primary spin fission process. further studies are needed to reveal which of these two hypotheses for formation of the paired binary systems is real. | asteroid pairs: a complex picture |
the current shock classification scheme of meteorites assigns shock levels of s1 (unshocked) to s6 (very strongly shocked) using shock effects in rock-forming minerals such as olivine and plagioclase. the s6 stage (55-90 gpa; 850-1750 °c) relies solely on localized effects in or near melt zones, the recrystallization of olivine, or the presence of mafic high-pressure phases such as ringwoodite. however, high whole rock temperatures and the presence of high-pressure phases that are unstable at those temperatures and pressures of zero gpa (e.g., ringwoodite) are two criteria that exclude each other. each type of high-pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase, and a maximum temperature of the whole rock after decompression to allow the preservation of this phase. rocks classified as s6 are characterized not by the presence but by the absence of those thermally unstable high-pressure phases. high-pressure phases in or attached to shock melt zones form mainly during shock pressure decline. this is because shocked rocks (<60 gpa) experience a shock wave with a broad isobaric pressure plateau only during low velocity (<4.5 km s-1) impacts, which rarely occur on small planetary bodies; e.g., the moon and asteroids. the mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock. | revising the shock classification of meteorites |
we present the target selection for the breakthrough listen search for extraterrestrial intelligence during the first year of observations at the green bank telescope, parkes telescope, and automated planet finder. on the way to observing 1,000,000 nearby stars in search of technological signals, we present three main sets of objects we plan to observe in addition to a smaller sample of exotica. we chose the 60 nearest stars, all within 5.1 pc from the sun. such nearby stars offer the potential to observe faint radio signals from transmitters that have a power similar to those on earth. we add a list of 1649 stars drawn from the hipparcos catalog that span the hertzprung-russell diagram, including all spectral types along the main sequence, subgiants, and giant stars. this sample offers diversity and inclusion of all stellar types, but with thoughtful limits and due attention to main sequence stars. our targets also include 123 nearby galaxies composed of a “morphological-type-complete” sample of the nearest spirals, ellipticals, dwarf spherioidals, and irregulars. while their great distances hamper the detection of technological electromagnetic radiation, galaxies offer the opportunity to observe billions of stars simultaneously and to sample the bright end of the technological luminosity function. we will also use the green bank and parkes telescopes to survey the plane and central bulge of the milky way. finally, the complete target list includes several classes of exotica, including white dwarfs, brown dwarfs, black holes, neutron stars, and asteroids in our solar system. | the breakthrough listen search for intelligent life: target selection of nearby stars and galaxies |
the pristine sample from the near-earth carbonaceous asteroid (162173) ryugu collected by the hayabusa2 spacecraft enabled us to analyze the pristine extraterrestrial material without uncontrolled exposure to the earth's atmosphere and biosphere. the initial analysis team for the soluble organic matter reported the detection of wide variety of organic molecules including racemic amino acids in the ryugu samples. here we report the detection of uracil, one of the four nucleobases in ribonucleic acid, in aqueous extracts from ryugu samples. in addition, nicotinic acid (niacin, a b3 vitamer), its derivatives, and imidazoles were detected in search for nitrogen heterocyclic molecules. the observed difference in the concentration of uracil between a0106 and c0107 may be related to the possible differences in the degree of alteration induced by energetic particles such as ultraviolet photons and cosmic rays. the present study strongly suggests that such molecules of prebiotic interest commonly formed in carbonaceous asteroids including ryugu and were delivered to the early earth. | uracil in the carbonaceous asteroid (162173) ryugu |
the japan aerospace exploration agency launched the asteroid sample return spacecraft "hayabusa2" on december 3, 2014. hayabusa2 will reach the c-type asteroid 162173 ryugu in 2018, and return back to the earth in 2020. sample collections from three sites, four surface rovers deployment and a 4 mj-class kinetic impact crater forming are planned in the 1.5 years of the asteroid-proximity operation. the mission objective of hayabusa2 has three aspects, science, engineering and exploration, all of which would be expanded by the successful round-trip journey. the objectives and technologies used in this mission is not a direct solution for the future planetary defense, but should contribute to this field by increasing general asteroid knowledge and enhancing human capabilities of small body-surface access/roving/sampling/impacting. this paper describes the outline of the hayabusa2 mission, overviews the kinetic impact technology as an example of planetary defense-related technologies and the current flight status after the two and a half years of the interplanetary cruise. | hayabusa2-sample return and kinetic impact mission to near-earth asteroid ryugu |
a graph class $\mathscr{c}$ is called monadically stable if one cannot interpret, in first-order logic, arbitrary large linear orders in colored graphs from $\mathscr{c}$. we prove that the model checking problem for first-order logic is fixed-parameter tractable on every monadically stable graph class. this extends the results of [grohe, kreutzer, and siebertz; j. acm '17] for nowhere dense classes and of [dreier, mählmann, and siebertz; stoc '23] for structurally nowhere dense classes to all monadically stable classes. as a complementary hardness result, we prove that for every hereditary graph class $\mathscr{c}$ that is edge-stable (excludes some half-graph as a semi-induced subgraph) but not monadically stable, first-order model checking is $\mathrm{aw}[*]$-hard on $\mathscr{c}$, and $\mathrm{w}[1]$-hard when restricted to existential sentences. this confirms, in the special case of edge-stable classes, an on-going conjecture that the notion of monadic nip delimits the tractability of first-order model checking on hereditary classes of graphs. for our tractability result, we first prove that monadically stable graph classes have almost linear neighborhood complexity. using this, we construct sparse neighborhood covers for monadically stable classes, which provides the missing ingredient for the algorithm of [dreier, mählmann, and siebertz; stoc '23]. the key component of this construction is the usage of orders with low crossing number [welzl; socg '88], a tool from the area of range queries. for our hardness result, we prove a new characterization of monadically stable graph classes in terms of forbidden induced subgraphs. we then use this characterization to show that in hereditary classes that are edge-stable but not monadically stable, one can effectively interpret the class of all graphs using only existential formulas. | first-order model checking on monadically stable graph classes |
1i/2017 u1 (‘oumuamua), a recently discovered asteroid in a hyperbolic orbit, is likely the first macroscopic object of extrasolar origin identified in the solar system. here, we present imaging and spectroscopic observations of ‘oumuamua using the palomar hale telescope as well as a search of meteor activity potentially linked to this object using the canadian meteor orbit radar. we find that ‘oumuamua exhibits a moderate spectral gradient of 10 % +/- 6 % {(100{nm})}-1, a value significantly lower than that of outer solar system bodies, indicative of a formation and/or previous residence in a warmer environment. imaging observation and spectral line analysis show no evidence that ‘oumuamua is presently active. negative meteor observation is as expected, since ejection driven by sublimation of commonly known cometary species such as co requires an extreme ejection speed of ∼40 m s-1 at ∼100 au in order to reach the earth. no obvious candidate stars are proposed as the point of origin for ‘oumuamua. given a mean free path of ∼109 ly in the solar neighborhood, ‘oumuamua has likely spent a very long time in interstellar space before encountering the solar system. | 1i/2017 u1 (‘oumuamua) is hot: imaging, spectroscopy, and search of meteor activity |
we study the origin of the interstellar object 1i/2017 u1 `oumuamua by juxtaposing estimates based on the observations with simulations. we speculate that objects like `oumuamua are formed in the debris disc as left over from the star and planet formation process, and subsequently liberated. the liberation process is mediated either by interaction with other stars in the parental star cluster, by resonant interactions within the planetesimal disc or by the relatively sudden mass loss when the host star becomes a compact object. integrating `oumuamua backward in time in the galactic potential together with stars from the gaia-tgas catalogue we find that about 1.3 myr ago `oumuamua passed the nearby star hip 17288 within a mean distance of 1.3 pc. by comparing nearby observed l-dwarfs with simulations of the galaxy, we conclude that the kinematics of `oumuamua is consistent with relatively young objects of 1.1-1.7 gyr. we just met `oumuamua by chance, and with a derived mean galactic density of ∼3 × 105 similarly sized objects within 100 au from the sun or ∼1014 per cubic parsec we expect about 2-12 such visitors per year within 1 au from the sun. | the origin of interstellar asteroidal objects like 1i/2017 u1 `oumuamua |
the environmental severity of large impacts on earth is influenced by their impact trajectory. impact direction and angle to the target plane affect the volume and depth of origin of vaporized target, as well as the trajectories of ejected material. the asteroid impact that formed the 66 ma chicxulub crater had a profound and catastrophic effect on earth's environment, but the impact trajectory is debated. here we show that impact angle and direction can be diagnosed by asymmetries in the subsurface structure of the chicxulub crater. comparison of 3d numerical simulations of chicxulub-scale impacts with geophysical observations suggests that the chicxulub crater was formed by a steeply-inclined (45-60° to horizontal) impact from the northeast; several lines of evidence rule out a low angle (<30°) impact. a steeply-inclined impact produces a nearly symmetric distribution of ejected rock and releases more climate-changing gases per impactor mass than either a very shallow or near-vertical impact. | a steeply-inclined trajectory for the chicxulub impact |
we present observations and analysis of the hostless and luminous type ia supernova 2022ilv, illustrating it is part of the 2003fg-like family, often referred to as super-chandrasekhar (ia-sc) explosions. the asteroid terrestrial-impact last alert system light curve shows evidence of a short-lived, pulse-like early excess, similar to that detected in another luminous type ia supernova (sn 2020hvf). the light curve is broad, and the early spectra are remarkably similar to those of sn 2009dc. adopting a redshift of z = 0.026 ± 0.005 for sn 2022ilv based on spectral matching, our model light curve requires a large 56ni mass in the range 0.7-1.5 m ⊙ and a large ejecta mass in the range 1.6-2.3 m ⊙. the early excess can be explained by fast-moving sn ejecta interacting with a thin, dense shell of circumstellar material close to the progenitor (~1013 cm) a few hours after the explosion. this may be realized in a double-degenerate scenario, wherein a white dwarf merger is preceded by the ejection of a small amount (~10-3-10-2 m ⊙) of hydrogen and helium-poor tidally stripped material. a deep pre-explosion pan-starrs1 stack indicates no host galaxy to a limiting magnitude of r ~ 24.5. this implies a surprisingly faint limit for any host of mr≳ -11, providing further evidence that these types of explosions occur predominantly in low-metallicity environments. | the luminous type ia supernova 2022ilv and its early excess emission |
the near-earth object (neo) surveyor mission is a nasa observatory designed to discover and characterize asteroids and comets. the mission's primary objective is to find the majority of objects large enough to cause severe regional impact damage (>140 m in effective spherical diameter) within its 5 yr baseline survey. operating at the sun-earth l1 lagrange point, the mission will survey to within 45° of the sun in an effort to find objects in the most earth-like orbits. the survey cadence is optimized to provide observational arcs long enough to distinguish near-earth objects from more distant small bodies that cannot pose an impact hazard reliably. over the course of its survey, neo surveyor will discover ~200,000-300,000 new neos down to sizes as small as ~10 m and thousands of comets, significantly improving our understanding of the probability of an earth impact over the next century. | the near-earth object surveyor mission |
the planar graph product structure theorem of dujmović, joret, micek, morin, ueckerdt, and wood [j. acm 2020] states that every planar graph is a subgraph of the strong product of a graph with bounded treewidth and a path. this result has been the key tool to resolve important open problems regarding queue layouts, nonrepetitive colourings, centered colourings, and adjacency labelling schemes. in this paper, we extend this line of research by utilizing shallow minors to prove analogous product structure theorems for several beyond planar graph classes. the key observation that drives our work is that many beyond planar graphs can be described as a shallow minor of the strong product of a planar graph with a small complete graph. in particular, we show that powers of planar graphs, $k$-planar, $(k,p)$-cluster planar, fan-planar and $k$-fan-bundle planar graphs have such a shallow-minor structure. using a combination of old and new results, we deduce that these classes have bounded queue-number, bounded nonrepetitive chromatic number, polynomial $p$-centred chromatic numbers, linear strong colouring numbers, and cubic weak colouring numbers. in addition, we show that $k$-gap planar graphs have at least exponential local treewidth and, as a consequence, cannot be described as a subgraph of the strong product of a graph with bounded treewidth and a path. | shallow minors, graph products and beyond planar graphs |
the extinction of the dinosaurs and around three-quarters of all living species was almost certainly caused by a large asteroid impact 66 million years ago. seismic data acquired across the impact site in mexico have provided spectacular images of the approximately 200-kilometre-wide chicxulub impact structure. in this review, we show how studying the impact site at chicxulub has advanced our understanding of formation of large craters and the environmental and palaeontological consequences of this impact. the chicxulub crater's asymmetric shape and size suggest an oblique impact and an impact energy of about 1023 joules, information that is important for quantifying the climatic effects of the impact. several thousand gigatonnes of asteroidal and target material were ejected at velocities exceeding 5 kilometres per second, forming a fast-moving cloud that transported dust, soot and sulfate aerosols around the earth within hours. these impact ejecta and soot from global wildfires blocked sunlight and caused global cooling, thus explaining the severity and abruptness of the mass extinction. however, it remains uncertain whether this impact winter lasted for many months or for more than a decade. further combined palaeontological and proxy studies of expanded cretaceous-palaeogene transitions should further constrain the climatic response and the precise cause and selectivity of the extinction. | the chicxulub impact and its environmental consequences |
using the osiris-rex mission and ground-based tracking data for the asteroid bennu, we derive new constraints on fifth forces and ultralight dark matter. the bounds we obtain are strongest for mediator masses $m \sim 10^{-18} - 10^{-17}\,{\rm ev}$, where we currently achieve the tightest bounds. our limits can be translated to a wide class of models leading to yukawa-type fifth forces, and we demonstrate how they apply to $u(1)_b$ dark photons and baryon-coupled scalars. our results demonstrate the potential of asteroid tracking in probing well-motivated extensions of the standard model and ultralight dark matter satisfying the fuzzy dark matter constraints. | constraints on fifth forces and ultralight dark matter from osiris-rex target asteroid bennu |
nine metal-polluted white dwarfs are observed with medium-resolution optical spectroscopy, where photospheric abundances are determined and interpreted by comparison with solar system objects. an improved method for making such comparisons is presented, which overcomes potential weaknesses of prior analyses, with numerous sources of error considered to highlight the limitations on interpretation. the stars are inferred to be accreting rocky, volatile-poor asteroidal materials with origins in differentiated bodies, in line with the consensus model. the most heavily polluted star in the sample has 14 metals detected, and appears to be accreting material from a rocky planetesimal, whose composition is mantle-like with a small fe-ni core component. some unusual abundances are present. one star is strongly depleted in ca, while two others show na abundances elevated above bulk-earth abundances; it is speculated that either the latter reflect diversity in the formation conditions of the source material, or they are traces of past accretion events. another star shows clear signs that accretion ceased around 5 myr ago, causing mg to dominate the photospheric abundances, as it has the longest diffusion time of the observed elements. observing such post-accretion systems allows constraints to be placed on models of the accretion process. | interpretation and diversity of exoplanetary material orbiting white dwarfs |
we obtained high-speed photometry of the disintegrating planetesimals orbiting the white dwarf wd 1145+017, spanning a period of four weeks. the light curves show a dramatic evolution of the system since the first observations obtained about seven months ago. multiple transit events are detected in every light curve, which have varying durations (≃3-12 minutes) and depths (≃10%-60%). the time-averaged extinction is ≃11%, much higher than at the time of the kepler observations. the shortest-duration transits require that the occulting cloud of debris has a few times the size of the white dwarf, longer events are often resolved into the superposition of several individual transits. the transits evolve on timescales of days, both in shape and in depth, with most of them gradually appearing and disappearing over the course of the observing campaign. several transits can be tracked across multiple nights, all of them recur on periods of ≃4.49 hr, indicating multiple planetary debris fragments on nearly identical orbits. identifying the specific origin of these bodies within this planetary system, and the evolution leading to their current orbits remains a challenging problem. | high-speed photometry of the disintegrating planetesimals at wd1145+017: evidence for rapid dynamical evolution |
an astroid-shaped loop of exceptional points (eps), comprising four cusps, is found to spawn from the triple degeneracy point in the brillouin zone (bz) of a lieb lattice with nearest-neighbor hoppings when non-hermiticity is introduced. the occurrence of the ep loop is due to the realness of the discriminant which is guaranteed by the non-hermitian chiral symmetry. the eps at the four cusps involve the coalescence of three eigenstates, which is the combined result of the non-hermitian chiral symmetry and mirror-t symmetry. the ep loop is exactly an astroid in the limit of an infinitesimal non-hermiticity. the ep loop expands from the m point with increasing non-hermiticity and splits into two ep loops at a critical non-hermiticity. the further increase of non-hermiticity contracts the two ep loops towards and finally to two eps at the x and y points in the bz, accompanied by the emergence of dirac-like cones. the two eps vanish at a larger non-hermiticity. the ep loop disappears and several discrete eps are found to survive when next-nearest hoppings are introduced to break the non-hermitian chiral symmetry. a topological invariant called the discriminant number is used to characterize their robustness against perturbations. both discrete eps and those on the ep loop(s) are found to show anisotropic asymptotic behaviors. finally, the experimental realization of the lieb lattice using a coupled waveguide array is discussed. | exceptional points make an astroid in non-hermitian lieb lattice: evolution and topological protection |
acoustic tunable focusing lens has important applications in acoustic fast imaging and three-dimensional displays. previously developed metasurface lenses suffer from narrow bandwidth and fixed geometric configuration, thereby limiting their further practical applications. here, we design an acoustic coding metasurface (acm) lens to achieve a broadband tunable focusing in air. this simple acm lens only includes two coding bits: a space-folding structure is designed to act as the bit '1' and an air unit with the same size is used as the bit '0'. by adjusting the coding sequence of the acm, the focus location of the acm lens can be modified efficiently and be steered in the whole working plane. moreover, the focusing behaviors of the acm lens are well performed in a large frequency range of ∼3.1 to ∼4.6 khz. our acm lens may be beneficial to applications that traditionally need different and complicated structures for tunable acoustic focusing. | broadband tunable focusing lenses by acoustic coding metasurfaces |
the discovery1 of 1i/2017 u1 (1i/`oumuamua) has provided the first glimpse of a planetesimal born in another planetary system. this interloper exhibits a variable colour within a range that is broadly consistent with local small bodies, such as the p- and d-type asteroids, jupiter trojans and dynamically excited kuiper belt objects2-7. 1i/`oumuamua appears unusually elongated in shape, with an axial ratio exceeding 5:1 (refs 1,4,5,8). rotation period estimates are inconsistent and varied, with reported values between 6.9 and 8.3 h (refs 4-6,9). here, we analyse all the available optical photometry data reported to date. no single rotation period can explain the exhibited brightness variations. rather, 1i/`oumuamua appears to be in an excited rotational state undergoing non-principal axis rotation, or tumbling. a satisfactory solution has apparent lightcurve frequencies of 0.135 and 0.126 h-1 and implies a longest-to-shortest axis ratio of ≳5:1, although the available data are insufficient to uniquely constrain the true frequencies and shape. assuming a body that responds to non-principal axis rotation in a similar manner to solar system asteroids and comets, the timescale to damp 1i/`oumuamua's tumbling is at least one billion years. 1i/`oumuamua was probably set tumbling within its parent planetary system and will remain tumbling well after it has left ours. | the tumbling rotational state of 1i/`oumuamua |
the only martian rock samples on earth are meteorites ejected from the surface of mars by asteroid impacts. the locations and geological contexts of the launch sites are currently unknown. determining the impact locations is essential to unravel the relations between the evolution of the martian interior and its surface. here we adapt a crater detection algorithm that compile a database of 90 million impact craters, allowing to determine the potential launch position of these meteorites through the observation of secondary crater fields. we show that tooting and 09-000015 craters, both located in the tharsis volcanic province, are the most likely source of the depleted shergottites ejected 1.1 million year ago. this implies that a major thermal anomaly deeply rooted in the mantle under tharsis was active over most of the geological history of the planet, and has sampled a depleted mantle, that has retained until recently geochemical signatures of mars' early history. | the tharsis mantle source of depleted shergottites revealed by 90 million impact craters |
the osiris-rex visible and infrared spectrometer (ovirs) is a point spectrometer covering the spectral range of 0.4 to 4.3 microns (25,000-2300 cm-1). its primary purpose is to map the surface composition of the asteroid bennu, the target asteroid of the osiris-rex asteroid sample return mission. the information it returns will help guide the selection of the sample site. it will also provide global context for the sample and high spatial resolution spectra that can be related to spatially unresolved terrestrial observations of asteroids. it is a compact, low-mass (17.8 kg), power efficient (8.8 w average), and robust instrument with the sensitivity needed to detect a 5% spectral absorption feature on a very dark surface (3% reflectance) in the inner solar system (0.89-1.35 au). it, in combination with the other instruments on the osiris-rex mission, will provide an unprecedented view of an asteroid's surface. | the osiris-rex visible and infrared spectrometer (ovirs): spectral maps of the asteroid bennu |
some asteroids eject dust, producing transient, comet-like comae and tails; these are the active asteroids. the causes of activity in this newly identified population are many and varied. they include impact ejection and disruption, rotational instabilities, electrostatic repulsion, radiation pressure sweeping, dehydration stresses, and thermal fracture, in addition to the sublimation of asteroidal ice. these processes were either unsuspected or thought to lie beyond the realm of observation before the discovery of asteroid activity. scientific interest in the active asteroids lies in their promise to open new avenues into the direct study of asteroid destruction, the production of interplanetary debris, the abundance of asteroid ice, and the origin of terrestrial planet volatiles. | the active asteroids |
dark matter may be in the form of nonbaryonic structures such as compact subhalos and boson stars. structures weighing between asteroid and solar masses may be discovered via gravitational microlensing, an astronomical probe that has in the past helped constrain the population of primordial black holes and baryonic massive astrophysical compact halo objects. we investigate the nontrivial effect of the size of and density distribution within these structures on the microlensing signal and constrain their populations using the eros-2 and ogle-iv surveys. structures larger than a solar radius are generally constrained more weakly than pointlike lenses, but stronger constraints may be obtained for structures with mass distributions that give rise to caustic crossings or produce larger magnifications. | gravitational microlensing by dark matter in extended structures |
context. the classification of the minor bodies of the solar system based on observables has been continuously developed and iterated over the past 40 yr. while prior iterations followed either the availability of large observational campaigns or new instrumental capabilities opening new observational dimensions, we see the opportunity to improve primarily upon the established methodology.aims: we developed an iteration of the asteroid taxonomy which allows the classification of partial and complete observations (i.e. visible, near-infrared, and visible-near-infrared spectrometry) and which reintroduces the visual albedo into the classification observables. the resulting class assignments are given probabilistically, enabling the uncertainty of a classification to be quantified.methods: we built the taxonomy based on 2983 observations of 2125 individual asteroids, representing an almost tenfold increase of sample size compared with the previous taxonomy. the asteroid classes are identified in a lower-dimensional representation of the observations using a mixture of common factor analysers model.results: we identify 17 classes split into the three complexes c, m, and s, including the new z-class for extremely-red objects in the main belt. the visual albedo information resolves the spectral degeneracy of the x-complex and establishes the p-class as part of the c-complex. we present a classification tool which computes probabilistic class assignments within this taxonomic scheme from asteroid observations, intrinsically accounting for degeneracies between classes based on the observed wavelength region. the taxonomic classifications of 6038 observations of 4526 individual asteroids are published.conclusions: the ability to classify partial observations and the reintroduction of the visual albedo into the classification provide a taxonomy which is well suited for the current and future datasets of asteroid observations, in particular provided by the gaia, mithneos, neo surveyor, and spherex surveys. the table of asteroid classifications and the templates of the defined taxonomic classes is only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/665/a26 | asteroid taxonomy from cluster analysis of spectrometry and albedo |
asteroids formed in a dynamically quiescent disk but their orbits became gravitationally stirred enough by jupiter to lead to high-speed collisions. as a result, many dozen large asteroids have been disrupted by impacts over the age of the solar system, producing groups of fragments known as asteroid families. here we explain how the asteroid families are identified, review their current inventory, and discuss how they can be used to get insights into long-term dynamics of main-belt asteroids. electronic tables of the membership for 122 notable families are reported on the planetary data system node. see related chapters in this volume for the significance of asteroid families for studies of physics of large-scale collisions, collisional history of the main belt, source regions of the near-earth asteroids, meteorites and dust particles, and space weathering. | identification and dynamical properties of asteroid families |
we analyze the trajectories of 313 particles seen in the near-bennu environment between december 2018 and september 2019. of these, 65% follow suborbital trajectories, 20% undergo more than one orbital revolution around the asteroid, and 15% directly escape on hyperbolic trajectories. the median lifetime of these particles is ∼6 hr. the trajectories are sensitive to bennu's gravitational field, which allows us to reliably estimate the spherical harmonic coefficients through degree 8 and to resolve nonuniform mass distribution through degree 3. the particles are perturbed by solar radiation pressure, enabling effective area-to-mass ratios to be estimated. by assuming that particles are oblate ellipsoids of revolution, and incorporating photometric measurements, we find a median axis ratio of 0.27 and diameters for equivalent-volume spheres ranging from 0.22-6.1 cm, with median 0.74 cm. our size distribution agrees well with that predicted for fragmentation due to diurnal thermal cycling. detailed models of known accelerations do not produce a match to the observed trajectories, so we also estimate empirical accelerations. these accelerations appear to be related to mismodeling of radiation pressure, but we cannot rule out contributions from mass loss. most ejections take place at local solar times in the afternoon and evening (12:00-24:00), although they occur at any time of day. we independently identify ten ejection events, some of which have previously been reported. we document a case where a particle ricocheted off the surface, revealing a coefficient of restitution 0.57±0.01 and demonstrating that some apparent ejections are not related to surface processes. | trajectory estimation for particles observed in the vicinity of (101955) bennu |
the question why non-avian dinosaurs went extinct 66 million years ago (ma) remains unresolved because of the coarseness of the fossil record. a sudden extinction caused by an asteroid is the most accepted hypothesis but it is debated whether dinosaurs were in decline or not before the impact. we analyse the speciation-extinction dynamics for six key dinosaur families, and find a decline across dinosaurs, where diversification shifted to a declining-diversity pattern ~76 ma. we investigate the influence of ecological and physical factors, and find that the decline of dinosaurs was likely driven by global climate cooling and herbivorous diversity drop. the latter is likely due to hadrosaurs outcompeting other herbivores. we also estimate that extinction risk is related to species age during the decline, suggesting a lack of evolutionary novelty or adaptation to changing environments. these results support an environmentally driven decline of non-avian dinosaurs well before the asteroid impact. | dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures |
the asteroid impact & deflection assessment (aida) mission will be the first space experiment to demonstrate asteroid impact hazard mitigation by using a kinetic impactor to deflect an asteroid. aida is an international cooperation, consisting of two mission elements: the nasa double asteroid redirection test (dart) mission and the esa asteroid impact mission (aim) rendezvous mission. the primary goals of aida are (i) to test our ability to perform a spacecraft impact on a potentially hazardous near-earth asteroid and (ii) to measure and characterize the deflection caused by the impact. the aida target will be the binary near-earth asteroid (65803) didymos, with the deflection experiment to occur in late september, 2022. the dart impact on the secondary member of the binary at 7 km/s is expected to alter the binary orbit period by about 4 minutes, assuming a simple transfer of momentum to the target, and this period change will be measured by earth-based observatories. the aim spacecraft will characterize the asteroid target and monitor results of the impact in situ at didymos. the dart mission is a full-scale kinetic impact to deflect a 150 m diameter asteroid, with known impactor conditions and with target physical properties characterized by the aim mission. predictions for the momentum transfer efficiency of kinetic impacts are given for several possible target types of different porosities, using housen and holsapple (2011) crater scaling model for impact ejecta mass and velocity distributions. results are compared to numerical simulation results using the smoothed particle hydrodynamics code of jutzi and michel (2014) with good agreement. the model also predicts that the ejecta from the dart impact may make didymos into an active asteroid, forming an ejecta coma that may be observable from earth-based telescopes. the measurements from aida of the momentum transfer from the dart impact, the crater size and morphology, and the evolution of an ejecta coma will substantially advance understanding of impact processes on asteroids. | asteroid impact & deflection assessment mission: kinetic impactor |
aims: we locate escape routes from the main asteroid belt, particularly into the near-earth-object (neo) region, and estimate the relative fluxes for different escape routes as a function of object size under the influence of the yarkovsky semimajor-axis drift.methods: we integrated the orbits of 78 355 known and 14 094 cloned main-belt objects and cybele and hilda asteroids (hereafter collectively called mbos) for 100 myr and recorded the characteristics of the escaping objects. the selected sample of mbos with perihelion distance q > 1.3 au and semimajor axis a < 4.1 au is essentially complete, with an absolute magnitude limit ranging from hv < 15.9 in the inner belt (a < 2.5 au) to hv < 14.4 in the outer belt (2.5 au < a < 4.1 au). we modeled the semimajor-axis drift caused by the yarkovsky force and assigned four different sizes (diameters of 0.1, 0.3, 1.0, and 3.0 km) and random spin obliquities (either 0 deg or 180 deg) for each test asteroid.results: we find more than ten obvious escape routes from the asteroid belt to the neo region, and they typically coincide with low-order mean-motion resonances with jupiter and secular resonances. the locations of the escape routes are independent of the semimajor-axis drift rate and thus are also independent of the asteroid diameter. the locations of the escape routes are likewise unaffected when we added a model for yarkovsky-o'keefe-radzievskii-paddack (yorp) cycles coupled with secular evolution of the rotation pole as a result of the solar gravitational torque. a yarkovsky-only model predicts a flux of asteroids entering the neo region that is too high compared to the observationally constrained flux, and the discrepancy grows larger for smaller asteroids. a combined yarkovsky and yorp model predicts a flux of small neos that is approximately a factor of 5 too low compared to an observationally constrained estimate. this suggests that the characteristic timescale of the yorp cycle is longer than our canonical yorp model predicts. | escape of asteroids from the main belt |
from the latest cretaceous (late campanian to maastrichtian, ~75-66 ma) to the earliest paleogene, fluctuations in greenhouse climate, inferred primarily from marine sediments, have been linked to volcanism, the chicxulub asteroid impact, and the cretaceous-paleogene (k-pg) mass extinction. in this paper, we summarize terrestrial climate records in mid-latitude east asia during the latest cretaceous and across the k-pg boundary, based on a multi-proxy approach from the geochronologically well-constrained sifangtai and mingshui formations (smf), accessed by scientific drilling of the songliao basin in northeastern china. evolution of sedimentary environments is characterized by five depositional units of fluvial-deltaic-lacustrine facies. development of four types of paleosols, including inceptisols, aridisols, vertisols and alfisols, is interpreted to primarily reflect climatic changes. correlations among sedimentary facies, paleosol features, illite chemistry index, chemical index of alteration, as well as stable and clumped isotopes of pedogenic carbonates and clay minerals of the smf validate their reliability for paleoclimate reconstruction, and indicate significant fluctuations in terrestrial climate and sedimentary environment. during global warming intervals possibly triggered by volcanism (e.g. ~69.5-68.5 ma), the songliao basin experienced a warmer and wetter climate with stronger terrestrial chemical weathering and more monsoon-derived moisture sourced from the pacific. in contrast, during global cooling intervals (e.g. ~70.5-69.5 ma and ~ 68.5-66.5 ma), the smf record a cooler and drier climate with less intensive chemical weathering and more westerlies-derived moisture. across the k-pg boundary, dramatic changes in land temperatures and hydroclimate correspond to the latest maastrichtian warming episode (~66.4-66.1 ma), the transient cooling preceding the k-pg boundary (~66.1-66.0 ma), and the earliest paleogene warming interval (~66.0-65.7 ma). temporal correlation of weathering index changes with the deccan traps volcanism suggests that volcanism and subsequent intensified weathering played a major role for climatic changes across the k-pg boundary. the integrated records of sedimentological and geochemical datasets from the songliao basin robustly demonstrate that the terrestrial climate of mid-latitude east asia responded strongly to greenhouse climate changes and to the catastrophic geological events from the latest cretaceous to the earliest paleogene. | terrestrial climate in mid-latitude east asia from the latest cretaceous to the earliest paleogene: a multiproxy record from the songliao basin in northeastern china |
over the past decade there has been a large increase in the number of automated camera networks that monitor the sky for fireballs. one of the goals of these networks is to provide the necessary information for linking meteorites to their pre-impact, heliocentric orbits and ultimately to their source regions in the solar system. we re-compute heliocentric orbits for the 25 meteorite falls published to date from original data sources. using these orbits, we constrain their most likely escape routes from the main asteroid belt and the cometary region by utilizing a state-of-the-art orbit model of the near-earth-object population, which includes a size-dependence in delivery efficiency. while we find that our general results for escape routes are comparable to previous work, the role of trajectory measurement uncertainty in escape-route identification is explored for the first time. moreover, our improved size-dependent delivery model substantially changes likely escape routes for several meteorite falls, most notably tagish lake which seems unlikely to have originated in the outer main belt as previously suggested. we find that reducing the uncertainty of fireball velocity measurements below ∼ 0.1 km/s does not lead to reduced uncertainties in the identification of their escape routes from the asteroid belt and, further, their ultimate source regions. this analysis suggests that camera networks should be optimized for the largest possible number of meteorite recoveries with measured speed precisions of order 0.1 km/s. | identification of meteorite source regions in the solar system |
a rapid accumulation of observations and interpretation has followed in the wake of 1i ‘oumuamua’s passage through the inner solar system. we briefly outline the consequences that this first detection of an interstellar asteroid implies for the planet-forming process, and we assess the near-term prospects for detecting and observing (both remotely and in situ) future solar system visitors of this type. drawing on detailed heat-transfer calculations that take both ‘oumuamua’s unusual shape and its chaotic tumbling into account, we affirm that the lack of a detectable coma in deep images of the object very likely arises from the presence of a radiation-modified coating of high molecular weight material (rather than a refractory bulk composition). assuming that ‘oumuamua is a typical representative of a larger population with a kinematic distribution similar to population i stars in the local galactic neighborhood, we calculate expected arrival rates, impact parameters, and velocities of similar objects and assess their prospects for detection using operational and forthcoming facilities. using ‘oumuamua as a proof of concept, we assess the prospects for missions that intercept interstellar objects using conventional chemical propulsion. using a “launch on detection” paradigm, we estimate wait times of order of 10 years between favorable mission opportunities with the detection capabilities of the large-scale synoptic survey telescope, a figure that will be refined as the population of interstellar asteroids becomes observationally better constrained. | the feasibility and benefits of in situ exploration of ‘oumuamua-like objects |
water consists of two most common chemical elements in the universe: hydrogen and oxygen. at the study of the solar and other planetary systems, water was found on planets, their satellites, in cometary nuclei, in asteroids, dwarf planets such as ceres and pluto. water also occurs in the giant molecular clouds at interstellar space, in the materials of protoplanetary disks, in the atmospheres of exoplanets. in addition, in liquid form, water can also be under the surface. most of the satellites of the giant planets also contain a huge amount of water ice. some satellites of saturn and jupiter even give evidence of the presence of oceans under their surface. these include, for example, enceladus, titan and dione in saturn; europe, ganymede and callisto near jupiter; here we will also include the satellite of neptune - triton. | water in solar system |
'oumuamua, the first bona fide interstellar planetesimal, was discovered passing through our solar system on a hyperbolic orbit. this object was likely dynamically ejected from an extrasolar planetary system after a series of close encounters with gas giant planets. to account for 'oumuamua's detection, simple arguments suggest that ∼1 m⊕ of planetesimals are ejected per solar mass of galactic stars. however, that value assumes mono-sized planetesimals. if the planetesimal mass distribution is instead top-heavy, the inferred mass in interstellar planetesimals increases to an implausibly high value. the tension between theoretical expectations for the planetesimal mass function and the observation of 'oumuamua can be relieved if a small fraction ({∼ } 0.1-1 {per cent}) of planetesimals are tidally disrupted on the pathway to ejection into 'oumuamua-sized fragments. using a large suite of simulations of giant planet dynamics including planetesimals, we confirm that 0.1-1 per cent of planetesimals pass within the tidal disruption radius of a gas giant on their pathway to ejection. 'oumuamua may thus represent a surviving fragment of a disrupted planetesimal. finally, we argue that an asteroidal composition is dynamically disfavoured for 'oumuamua, as asteroidal planetesimals are both less abundant and ejected at a lower efficiency than cometary planetesimals. | implications of the interstellar object 1i/'oumuamua for planetary dynamics and planetesimal formation |
compared with previous space-borne surveys, the transiting exoplanet survey satellite (tess) provides a unique and new approach to observe solar system objects. while its primary mission avoids the vicinity of the ecliptic plane by approximately six degrees, the scale height of the solar system debris disk is large enough to place various small body populations in the field of view. in this paper we present the first data release of photometric analysis of tess observations of small solar system bodies, focusing on the bright end of the observed main-belt asteroid and jovian trojan populations. this data release, named tssys-dr1, contains 9912 light curves obtained and extracted in a homogeneous manner, and triples the number of bodies with unambiguous fundamental rotation characteristics, namely where accurate periods and amplitudes are both reported. our catalog clearly shows that the number of bodies with long rotation periods are definitely underestimated by all previous ground-based surveys, by at least an order of magnitude. | solar system objects observed with tess—first data release: bright main-belt and trojan asteroids from the southern survey |
graph neural networks (gnns) extend basic neural networks (nns) by using graph structures based on the relational inductive bias (homophily assumption). while gnns have been commonly believed to outperform nns in real-world tasks, recent work has identified a non-trivial set of datasets where their performance compared to nns is not satisfactory. heterophily has been considered the main cause of this empirical observation and numerous works have been put forward to address it. in this paper, we first revisit the widely used homophily metrics and point out that their consideration of only graph-label consistency is a shortcoming. then, we study heterophily from the perspective of post-aggregation node similarity and define new homophily metrics, which are potentially advantageous compared to existing ones. based on this investigation, we prove that some harmful cases of heterophily can be effectively addressed by local diversification operation. then, we propose the adaptive channel mixing (acm), a framework to adaptively exploit aggregation, diversification and identity channels node-wisely to extract richer localized information for diverse node heterophily situations. acm is more powerful than the commonly used uni-channel framework for node classification tasks on heterophilic graphs and is easy to be implemented in baseline gnn layers. when evaluated on 10 benchmark node classification tasks, acm-augmented baselines consistently achieve significant performance gain, exceeding state-of-the-art gnns on most tasks without incurring significant computational burden. | revisiting heterophily for graph neural networks |
we overview various efforts within the dart investigation team's ejecta working group to predict the characteristics, quantity, dynamical behavior, and observability of dart impact ejecta. we discuss various methodologies for simulation of the impact/cratering process with their advantages and drawbacks in relation to initializing ejecta for subsequent dynamical propagation through and away from the didymos system. we discuss the most relevant forces acting on ejecta once decoupled from dimorphos's surface and highlight various software packages we have developed and used to dynamically simulate ejecta under the action of those forces. with some additional software packages, we explore the influence of additional perturbing effects, such as interparticle collisions within true n-body codes and nonspherical and rotating particles' interplay with solar radiation pressure. we find that early-timescale and close-proximity ejecta evolution is highly sensitive to some of these effects (e.g., collisions) while relatively insensitive to other factors. we present a methodology for turning the time-evolving size- and spatially discretized number density field output from ejecta simulations into synthetic images for multiple platforms/cameras over wide-ranging vantage points and timescales. we present such simulated images and apply preliminary analyses to them for nominal and off-nominal cases bracketing realistic total mass of ejecta and ejecta cumulative size-frequency distribution slope. our analyses foreshadow the information content we may be able to extract from the actual images taken during and after the dart encounter by both liciacube and earth-vicinity telescopes. | pre-encounter predictions of dart impact ejecta behavior and observability |
the catastrophic disruption of the l chondrite parent body in the asteroid belt c. 470 ma initiated a prolonged meteorite bombardment of earth that started in the ordovician and continues today. abundant l chondrite meteorites in middle ordovician strata have been interpreted to be the consequence of the asteroid breakup event. here we report a zircon u-pb date of 467.50+/-0.28 ma from a distinct bed within the meteorite-bearing interval of southern sweden that, combined with published cosmic-ray exposure ages of co-occurring meteoritic material, provides a precise age for the l chondrite breakup at 468.0+/-0.3 ma. the new zircon date requires significant revision of the ordovician timescale that has implications for the understanding of the astrogeobiologic development during this period. it has been suggested that the middle ordovician meteorite bombardment played a crucial role in the great ordovician biodiversification event, but this study shows that the two phenomena were unrelated. | refined ordovician timescale reveals no link between asteroid breakup and biodiversification |
in the scenario in which qcd axion dark matter is produced after inflation, the universe is populated by large inhomogeneities on very small scales. eventually, these fluctuations will collapse gravitationally to form dense axion miniclusters that trap up to ∼75 % of the dark matter within asteroid-mass clumps. axion miniclusters are physically tiny however, so haloscope experiments searching for axions directly on earth are much more likely to be probing "minivoids"—the space in between miniclusters. this scenario seems like it ought to spell doom for haloscopes, but while these minivoids might be underdense, they are not totally devoid of axions. using schrödinger-poisson and n -body simulations to evolve from realistic initial field configurations, we quantify the extent to which the local ambient dark matter density is suppressed in the postinflationary scenario. we find that a typical experimental measurement will sample an axion density that is only around 10% of the expected galactic dark matter density. our results are taken as conservative estimates and have implications for experimental campaigns lasting longer than a few years, as well as broadband haloscopes that have sensitivity to transient signatures. we show that for a o (year )-long integration times, the measured dark matter density should be expected to vary by 20%-30%. | axion minivoids and implications for direct detection |
the first discovered interstellar object (iso), `oumuamua (1i/2017 u1) shows a dry and rocky surface, an unusually elongated shape, with short-to-long axis ratio c∕a ≲ 1∕6, a low velocity relative to the local standard of rest (~10 km s-1), non-gravitational accelerations and tumbles on a timescale of a few hours1-9. the inferred number density (~3.5 × 1013-2 × 1015 pc-3) for a population of asteroidal isos10,11 outnumbers cometary isos12 by ≥103, in contrast to the much lower ratio (≲10-2) of rocky/icy kuiper belt objects13. although some scenarios can cause the ejection of asteroidal isos14,15, a unified formation theory has yet to comprehensively link all `oumuamua's puzzling characteristics and to account for the population. here we show by numerical simulations that `oumuamua-like isos can be prolifically produced through extensive tidal fragmentation and ejected during close encounters of their volatile-rich parent bodies with their host stars. material strength enhanced by the intensive heating during periastron passages enables the emergence of extremely elongated triaxial isos with shape c∕a ≲ 1∕10, sizes a ≈ 100 m and rocky surfaces. although volatiles with low sublimation temperature (such as co) are concurrently depleted, h2o buried under surfaces is preserved in these isos, providing an outgassing source without measurable cometary activities for `oumuamua's non-gravitational accelerations during its passage through the inner solar system. we infer that the progenitors of `oumuamua-like isos may be kilometre-sized long-period comets from oort clouds, kilometre-sized residual planetesimals from debris disks or planet-sized bodies at a few astronomical units, orbiting around low-mass main-sequence stars or white dwarfs. these provide abundant reservoirs to account for `oumuamua's occurrence rate. | tidal fragmentation as the origin of 1i/2017 u1 (`oumuamua) |
the international rosetta mission was launched in 2004 and consists of the orbiter spacecraft rosetta and the lander philae. the aim of the mission is to map the comet 67p/churyumov-gerasimenko by remote sensing, and to examine its environment in situ and its evolution in the inner solar system. rosetta was the first spacecraft to rendezvous with and orbit a comet, accompanying it as it passes through the inner solar system, and to deploy a lander, philae, and perform in situ science on the comet's surface. the primary goals of the mission were to: characterize the comet's nucleus; examine the chemical, mineralogical and isotopic composition of volatiles and refractories; examine the physical properties and interrelation of volatiles and refractories in a cometary nucleus; study the development of cometary activity and the processes in the surface layer of the nucleus and in the coma; detail the origin of comets, the relationship between cometary and interstellar material and the implications for the origin of the solar system; and characterize asteroids 2867 steins and 21 lutetia. this paper presents a summary of mission operations and science, focusing on the rosetta orbiter component of the mission during its comet phase, from early 2014 up to september 2016. this article is part of the themed issue 'cometary science after rosetta'. | the rosetta mission orbiter science overview: the comet phase |
we have obtained extensive photometric observations of the polluted white dwarf wd 1145+017 which has been reported to be transited by at least one, and perhaps several, large asteroids with dust emission. observation sessions on 37 nights spanning 2015 november to 2016 january with small to modest size telescopes have detected 237 significant dips in flux. periodograms reveal a significant periodicity of 4.5004 h consistent with the dominant (`a') period detected with k2. the folded light curve shows an hour-long depression in flux with a mean depth of nearly 10 per cent. this depression is, in turn, comprised of a series of shorter and sometimes deeper dips which would be unresolvable with k2. we also find numerous dips in flux at other orbital phases. nearly all of the dips associated with this activity appear to drift systematically in phase with respect to the `a' period by about 2.5 min d-1 with a dispersion of ∼0.5 min d-1, corresponding to a mean drift period of 4.4928 h. we are able to track ∼15 discrete drifting features. the `b'-`f' periods found with k2 are not detected, but we would not necessarily have expected to see them. we explain the drifting motion as due to smaller fragmented bodies that break off from the asteroid and go into a slightly smaller orbit. in this interpretation, we can use the drift rate to determine the mass of the asteroid, which we find to be ≈1023 g, or about 1/10th the mass of ceres. | drifting asteroid fragments around wd 1145+017 |
meso-cenozoic evidence suggests links between changes in the expression of orbital changes and millennia-scale climatic- and biotic variations, but proof for such shifts in orbital cyclicity farther back in geological time is lacking. here, we report a 469-million-year-old palaeozoic energy transfer from precession to 405 kyr eccentricity cycles that coincides with the start of the great ordovician biodiversification event (gobe). based on an early middle ordovician astronomically calibrated cyclostratigraphic framework we find this orbital change to succeed the onset of icehouse conditions by 200,000 years, suggesting a climatic origin. recently, this icehouse was postulated to be facilitated by extra-terrestrial dust associated with an asteroid breakup. our timescale, however, shows the meteor bombardment to post-date the icehouse by 800,000 years, instead pausing the gobe 600,000 years after its initiation. resolving milankovitch cyclicity in deep time thus suggests universal orbital control in modulating climate, and maybe even biodiversity accumulation, through geological time. | middle ordovician astrochronology decouples asteroid breakup from glacially-induced biotic radiations |
the optical navigation camera telescope (onc-t) is a telescopic framing camera with seven colors onboard the hayabusa2 spacecraft launched on december 3, 2014. the main objectives of this instrument are to optically navigate the spacecraft to asteroid ryugu and to conduct multi-band mapping the asteroid. we conducted performance tests of the instrument before its installation on the spacecraft. we evaluated the dark current and bias level, obtained data on the dependency of the dark current on the temperature of the charge-coupled device (ccd). the bias level depends strongly on the temperature of the electronics package but only weakly on the ccd temperature. the dark-reference data, which is obtained simultaneously with observation data, can be used for estimation of the dark current and bias level. a long front hood is used for onc-t to reduce the stray light at the expense of flatness in the peripheral area of the field of view (fov). the central area in fov has a flat sensitivity, and the limb darkening has been measured with an integrating sphere. the onc-t has a wheel with seven bandpass filters and a panchromatic glass window. we measured the spectral sensitivity using an integrating sphere and obtained the sensitivity of all the pixels. we also measured the point-spread function using a star simulator. measurement results indicate that the full width at half maximum is less than two pixels for all the bandpass filters and in the temperature range expected in the mission phase except for short periods of time during touchdowns. | preflight calibration test results for optical navigation camera telescope (onc-t) onboard the hayabusa2 spacecraft |
white dwarfs (wds) have atmospheres that are expected to consist nearly entirely of hydrogen and helium, since heavier elements will sink out of sight on short timescales. however, observations have revealed atmospheric pollution by heavier elements in about a quarter to a half of all wds. while most of the pollution can be accounted for with asteroidal or dwarf planetary material, recent observations indicate that larger planetary bodies, as well as icy and volatile material from kuiper belt analog objects, are also viable sources of pollution. the commonly accepted pollution mechanisms, namely scattering interactions between planetary bodies orbiting the wds, can hardly account for pollution by objects with large masses or long-period orbits. here we report on a mechanism that naturally leads to the emergence of massive body and icy and volatile material pollution. this mechanism occurs in wide binary stellar systems, where the mass loss of the planets’ host stars during post main sequence stellar evolution can trigger the eccentric kozai-lidov mechanism. this mechanism leads to large eccentricity excitations, which can bring massive and long-period objects close enough to the wds to be accreted. we find that this mechanism readily explains and is consistent with observations. | throwing icebergs at white dwarfs |
the hot water and acid extracts of two different ryugu samples collected by the hayabusa2 mission were analyzed for the presence of aliphatic amines and amino acids. the abundances and relative distributions of both classes of molecules were determined, as well as the enantiomeric compositions of the chiral amino acids. the ryugu samples studied here were recovered from sample chambers a and c, which were composed of surface material, and a combination of surface and possible subsurface material, respectively. a total of thirteen amino acids were detected and quantitated in these samples, with an additional five amino acids that were tentatively identified but not quantitated. the abundances of four aliphatic amines identified in the ryugu samples were also determined in the current work. amino acids were observed in the acid hydrolyzed and unhydrolyzed hot water extracts of asteroid ryugu regolith using liquid chromatography with uv fluorescence detection and high-resolution mass spectrometry. conversely, aliphatic amines were only analyzed in the unhydrolyzed hot water ryugu extracts. two- to six-carbon (c2-c6) amino acids with individual abundances ranging from 0.02 to 15.8 nmol g-1, and one- to three-carbon (c1-c3) aliphatic amines with individual abundances from 0.05 to 34.14 nmol g-1, were found in the hot water extracts. several non-protein amino acids that are rare in biology, including β-amino-n-butyric acid (β-aba) and β-aminoisobuytric acid (β-aib), were racemic or very nearly racemic, thus indicating their likely abiotic origins. trace amounts of select protein amino acids that were enriched in the l-enantiomer may indicate low levels of terrestrial amino acid contamination in the samples. however, the presence of elevated abundances of free and racemic alanine, a common protein amino acid in terrestrial biology, and elevated abundances of the predominately free and racemic non-protein amino acids, β-aba and β-aib, indicate that many of the amino acids detected in the ryugu water extracts were indigenous to the samples. although the ryugu samples have been found to be chemically similar to ci type carbonaceous chondrites, the measured concentrations and relative distributions of amino acids and aliphatic amines in ryugu samples were notably different from those previously observed for the ci1.1 carbonaceous chondrite, orgueil. this discrepancy could be the result of differences in the original chemical compositions of the parent bodies and/or alteration conditions, such as space weathering. in addition to α-amino acids that could have been formed by strecker cyanohydrin synthesis during a low temperature aqueous alteration phase, β-, γ-, and δ-amino acids, including c3 - c5 straight-chain n-ω-amino acids that are not formed by strecker synthesis, were also observed in the ryugu extracts. the suite of amino acids measured in the ryugu samples indicates that multiple amino acid formation mechanisms were active on the ryugu parent body. the analytical techniques used here are well-suited to search for similar analytes in asteroid bennu material collected by the nasa osiris-rex mission scheduled for earth return in september 2023. | extraterrestrial amino acids and amines identified in asteroid ryugu samples returned by the hayabusa2 mission |
samples of the carbonaceous asteroid (162173) ryugu were collected and brought to earth by the hayabusa2 spacecraft. we investigated the macromolecular organic matter in ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. the spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). the morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. the diversity of the organic matter indicates variable levels of aqueous alteration on ryugu’s parent body. | macromolecular organic matter in samples of the asteroid (162173) ryugu |
identifying and extracting building boundaries from remote sensing data has been one of the hot topics in photogrammetry for decades. the active contour model (acm) is a robust segmentation method that has been widely used in building boundary extraction, but which often results in biased building boundary extraction due to tree and background mixtures. although the classification methods can improve this efficiently by separating buildings from other objects, there are often ineluctable salt and pepper artifacts. in this paper, we combine the robust classification convolutional neural networks (cnn) and acm to overcome the current limitations in algorithms for building boundary extraction. we conduct two types of experiments: the first integrates acm into the cnn construction progress, whereas the second starts building footprint detection with a cnn and then uses acm for post processing. three level assessments conducted demonstrate that the proposed methods could efficiently extract building boundaries in five test scenes from two datasets. the achieved mean accuracies in terms of the f1 score for the first type (and the second type) of the experiment are 96.43 ± 3.34% (95.68 ± 3.22%), 88.60 ± 3.99% (89.06 ± 3.96%), and 91.62 ±1.61% (91.47 ± 2.58%) at the scene, object, and pixel levels, respectively. the combined cnn and acm solutions were shown to be effective at extracting building boundaries from high-resolution optical images and lidar data. | extracting building boundaries from high resolution optical images and lidar data by integrating the convolutional neural network and the active contour model |
an analysis of the surface and interior state of asteroid (101955) bennu, the target asteroid of the osiris-rex sample return mission, is given using models based on earth-based observations of this body. these observations have enabled models of its shape, spin state, mass and surface properties to be developed. based on these data the range of surface and interior states possible for this body are evaluated, assuming a uniform mass distribution. these products include the geopotential, surface slopes, near-surface dynamical environment, interior stress states and other quantities of interest. in addition, competing theories for its current shape are reviewed along with the relevant planned osiris-rex measurements. | the geophysical environment of bennu |
on asteroids, fractures develop due to stresses driven by diurnal temperature variations at spatial scales ranging from sub-millimetres to metres. however, the timescales of such rock fracturing by thermal fatigue are poorly constrained by observations. here we analyse images of the asteroid (101955) bennu obtained by the origins, spectral interpretation, resource identification and security-regolith explorer (osiris-rex) mission and show that metre-scale fractures on the boulders exposed at the surface have a preferential meridional orientation, consistent with cracking induced by diurnal temperature variations. using an analytical model of fracture propagation, we suggest that fractures the length of those on bennu's boulders can be produced in 104-105 years. this is a comparable or shorter timescale than mass movement processes that act to expose fresh surfaces and reorient boulders and any preferential direction signature. we propose that boulder surface fracturing happens rapidly compared with the lifetime in near-earth space of bennu and other carbonaceous asteroids. the damage due to this space-weathering process has consequences for the material properties of these asteroids, with implications for the preservation of the primordial signature acquired during the accretional phases in the protoplanetary disk of our solar system. | alignment of fractures on bennu's boulders indicative of rapid asteroid surface evolution |
the lijiang 2.4-meter telescope (ljt), the largest common-purpose optical telescope in china, has been available to the worldwide astronomical community since 2008. it is located at the gaomeigu site, lijiang observatory (ljo), in the southwest of china. the site has very good observational conditions. during its 10-year operation, several instruments have been equipped on the ljt. astronomers can perform both photometric and spectral observations. the main scientific goals of ljt include recording photometric and spectral evolution of supernovae, reverberation mapping of active galactic nuclei, investigating the physical properties of binary stars and near-earth objects (comets and asteroids), and identification of exoplanets and all kinds of transients. until now, the masses of 41 high accretion rate black holes have been measured, and more than 168 supernovae have been identified by the ljt. more than 190 papers related to the ljt have been published. in this paper, the general observation conditions of the gaomeigu site is introduced at first. then, the structure of the ljt is described in detail, including the optical, mechanical, motion and control system. the specification of all the instruments and some detailed parameters of the yfosc is also presented. finally, some important scientific results and future expectations are summarized. | lijiang 2.4-meter telescope and its instruments |
we present the first attempt to fit the light curve of the interstellar visitor 'oumuamua using a physical model that includes optional torque. we consider both conventional (lommel-seeliger triaxial ellipsoid) and alternative ('black-and-white ball', 'solar sail') brightness models. with all the brightness models, some torque is required to explain the timings of the most conspicuous features - deep minima - of the asteroid's light curve. our best-fitting models are a thin disc (aspect ratio 1:6) and a thin cigar (aspect ratio 1:8) that are very close to being axially symmetric. both models are tumbling and require some torque that has the same amplitude in relation to 'oumuamua's linear non-gravitational acceleration as in solar system comets whose dynamics is affected by outgassing. assuming random orientation of the angular momentum vector, we compute probabilities for our best-fitting models. we show that cigar-shaped models suffer from a fine-tuning problem and have only 16 per cent probability to produce light-curve minima as deep as the ones present in 'oumuamua's light curve. disc-shaped models, on the other hand, are very likely (at 91 per cent) to produce minima of the required depth. from our analysis, the most likely model for 'oumuamua is a thin disc (slab) experiencing moderate torque from outgassing. | modelling the light curve of `oumuamua: evidence for torque and disc-like shape |
nasa's new horizons mission performed the first flyby of a small kuiper belt object (kbo), (486958) arrokoth on 1 january 2019. the fast flyby revealed a fascinating, flattened, contact binary replete with a variety of unexpected geologic terrains. however, the irregular shape and constraints imposed by the fast flyby makes it a challenge to understand these features. here we use the latest new horizons shape models of arrokoth to investigate its geophysical environment, including its surface slopes, gravity field, and moments of inertia—which are critical context for understanding arrokoth's formation, evolution, and peculiar geology. we find that arrokoth's surface features have a complicated relationship to its geophysical environment. for example, bright material tends to be concentrated in geopotential lows (like the neck), consistent with mass wasting—however, this trend is not consistently observed across arrokoth. mass wasting may naturally explain some aspects of arrokoth's geology, but the actual dynamics of material transport may be complicated owing to arrokoth's unique shape, spin-rate, and inferred density. while new horizons's fast and distant flyby precluded directly measuring arrokoth's mass, we used techniques previously pioneered for comets and asteroids to infer its density. we find that arrokoth has a low bulk density of ρ = 235 kg/m3 (1σ range: 155-600 kg/m3). this density is low compared to previously explored small bodies, but is comparable to comets, select binary kbos, and the ring-moons of saturn. this low density may be a critical data-point for understanding the formation of planetesimals at the dawn of the solar system. | the geophysical environment of (486958) arrokoth—a small kuiper belt object explored by new horizons |
the osiris-rex camera suite (ocams) onboard the osiris-rex spacecraft is used to study the shape and surface of the mission's target, asteroid (101955) bennu, in support of the selection of a sampling site. we present calibration methods and results for the three ocams cameras—mapcam, polycam, and samcam—using data from pre-flight and in-flight calibration campaigns. pre-flight calibrations established a baseline for a variety of camera properties, including bias and dark behavior, flat fields, stray light, and radiometric calibration. in-flight activities updated these calibrations where possible, allowing us to confidently measure bennu's surface. accurate calibration is critical not only for establishing a global understanding of bennu, but also for enabling analyses of potential sampling locations and for providing scientific context for the returned sample. | ground and in-flight calibration of the osiris-rex camera suite |
pebble accretion is an efficient mechanism that is able to build up the core of the giant planets within the lifetime of the protoplanetary disc gas-phase. the core grows via this process until the protoplanet reaches its pebble isolation mass and starts to accrete gas. during the growth, the protoplanet undergoes a rapid, large-scale, inward migration due to the interactions with the gaseous protoplanetary disc. in this work, we have investigated how this early migration would have affected the minor body populations in our solar system. in particular, we focus on the jupiter trojan asteroids (bodies in the coorbital resonance 1:1 with jupiter, librating around the l4 and l5 lagrangian points called, respectively, the leading and the trailing swarm) and the hilda asteroids. we characterised their orbital parameter distributions after the disc dispersal and their formation location and compare them to the same populations produced in a classical in situ growth model. we find that a massive and eccentric hilda group is captured during the migration from a region between 5 and 8 au and subsequently depleted during the late instability of the giant planets. our simulations also show that inward migration of the giant planets always produces a jupiter trojans' leading swarm more populated than the trailing one, with a ratio comparable to the current observed trojan asymmetry ratio. the in situ formation of jupiter, on the other hand, produces symmetric swarms. the reason for the asymmetry is the relative drift between the migrating planet and the particles in the coorbital resonance. the capture happens during the growth of jupiter's core and trojan asteroids are afterwards carried along during the giant planet's migration to their final orbits. the asymmetry and eccentricity of the captured trojans correspond well to observations, but their inclinations are near zero and their total mass is three to four orders of magnitude higher than the current population. future modelling will be needed to understand whether the dynamical evolution of the trojans over billions of years will raise the inclinations and deplete the masses to observed values. | consequences of planetary migration on the minor bodies of the early solar system |
with the advances in sensor technology, big data, and artificial intelligence, unobtrusive in-home health monitoring has been a research focus for decades. following up our research on smart vehicles, within the framework of unobtrusive health monitoring in private spaces, this work attempts to provide a guide to current sensor technology for unobtrusive in-home monitoring by a literature review of the state of the art and to answer, in particular, the questions: (1) what types of sensors can be used for unobtrusive in-home health data acquisition? (2) where should the sensors be placed? (3) what data can be monitored in a smart home? (4) how can the obtained data support the monitoring functions? we conducted a retrospective literature review and summarized the state-of-the-art research on leveraging sensor technology for unobtrusive in-home health monitoring. for structured analysis, we developed a four-category terminology (location, unobtrusive sensor, data, and monitoring functions). we acquired 912 unique articles from four relevant databases (acm digital lib, ieee xplore, pubmed, and scopus) and screened them for relevance, resulting in n=55 papers analyzed in a structured manner using the terminology. the results delivered 25 types of sensors (motion sensor, contact sensor, pressure sensor, electrical current sensor, etc.) that can be deployed within rooms, static facilities, or electric appliances in an ambient way. while behavioral data (e.g., presence (n=38), time spent on activities (n=18)) can be acquired effortlessly, physiological parameters (e.g., heart rate, respiratory rate) are measurable on a limited scale (n=5). behavioral data contribute to functional monitoring. emergency monitoring can be built up on behavioral and environmental data. acquired physiological parameters allow reasonable monitoring of physiological functions to a limited extent. environmental data and behavioral data also detect safety and security abnormalities. social interaction monitoring relies mainly on direct monitoring of tools of communication (smartphone; computer). in summary, convincing proof of a clear effect of these monitoring functions on clinical outcome with a large sample size and long-term monitoring is still lacking. | unobtrusive health monitoring in private spaces: the smart home |
understanding the true nature of extra-terrestrial water and organic matter that were present at the birth of our solar system, and their subsequent evolution, necessitates the study of pristine astromaterials. in this study, we have studied both the water and organic contents from a dust particle recovered from the surface of near-earth asteroid 25143 itokawa by the hayabusa mission, which was the first mission that brought pristine asteroidal materials to earth's astromaterial collection. the organic matter is presented as both nanocrystalline graphite and disordered polyaromatic carbon with high d/h and 15n/14n ratios (δd = + 4868 ± 2288‰; δ15n = + 344 ± 20‰) signifying an explicit extra-terrestrial origin. the contrasting organic feature (graphitic and disordered) substantiates the rubble-pile asteroid model of itokawa, and offers support for material mixing in the asteroid belt that occurred in scales from small dust infall to catastrophic impacts of large asteroidal parent bodies. our analysis of itokawa water indicates that the asteroid has incorporated d-poor water ice at the abundance on par with inner solar system bodies. the asteroid was metamorphosed and dehydrated on the formerly large asteroid, and was subsequently evolved via late-stage hydration, modified by d-enriched exogenous organics and water derived from a carbonaceous parent body. | organic matter and water from asteroid itokawa |
we present the first good evidence for exocomet transits of a host star in continuum light in data from the kepler mission. the kepler star in question, kic 3542116, is of spectral type f2v and is quite bright at kp = 10. the transits have a distinct asymmetric shape with a steeper ingress and slower egress that can be ascribed to objects with a trailing dust tail passing over the stellar disc. there are three deeper transits with depths of ≃ 0.1 per cent that last for about a day, and three that are several times more shallow and of shorter duration. the transits were found via an exhaustive visual search of the entire kepler photometric data set, which we describe in some detail. we review the methods we use to validate the kepler data showing the comet transits, and rule out instrumental artefacts as sources of the signals. we fit the transits with a simple dust-tail model, and find that a transverse comet speed of ∼35-50 km s-1 and a minimum amount of dust present in the tail of ∼1016 g are required to explain the larger transits. for a dust replenishment time of ∼10 d, and a comet lifetime of only ∼300 d, this implies a total cometary mass of ≳3 × 1017 g, or about the mass of halley's comet. we also discuss the number of comets and orbital geometry that would be necessary to explain the six transits detected over the 4 yr of kepler prime-field observations. finally, we also report the discovery of a single comet-shaped transit in kic 11084727 with very similar transit and host-star properties. | likely transiting exocomets detected by kepler |
this review addresses our current understanding of comets that venture close to the sun, and are hence exposed to much more extreme conditions than comets that are typically studied from earth. the extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. we propose clear definitions for these comets: we use the term near-sun comets to encompass all objects that pass sunward of the perihelion distance of planet mercury (0.307 au). sunskirters are defined as objects that pass within 33 solar radii of the sun's centre, equal to half of mercury's perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid roche limit. finally, comets with orbits that intersect the solar photosphere are termed sundivers. we summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. relevant comet populations are described, including the kreutz, marsden, kracht, and meyer groups, near-sun asteroids, and a brief discussion of their origins. the importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. the physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. the different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-sun environment. comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. we address the relevance of work on comets near the sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics. | the science of sungrazers, sunskirters, and other near-sun comets |
we observed the newly discovered hyperbolic minor planet 1i/‘oumuamua (2017 u1) on 2017 october 30 with lowell observatory’s 4.3 m discovery channel telescope. from these observations, we derived a partial lightcurve with a peak-to-trough amplitude of at least 1.2 mag. this lightcurve segment rules out rotation periods less than 3 hr and suggests that the period is at least 5 hr. on the assumption that the variability is due to a changing cross-section, the axial ratio is at least 3:1. we saw no evidence for a coma or tail in either individual images or in a stacked image having an equivalent exposure time of 9000 s. | on the rotation period and shape of the hyperbolic asteroid 1i/‘oumuamua (2017 u1) from its lightcurve |
near-earth objects (neos) are a transient population of small bodies with orbits near or in the terrestrial planet region. they represent a mid-stage in the dynamical cycle of asteroids and comets, which starts with their removal from the respective source regions-the main belt and trans-neptunian scattered disk-and ends as bodies impact planets, disintegrate near the sun, or are ejected from the solar system. here we develop a new orbital model of neos by numerically integrating asteroid orbits from main-belt sources and calibrating the results on observations of the catalina sky survey. the results imply a size-dependent sampling of the main belt with the ν 6 and 3:1 resonances producing ≃30% of neos with absolute magnitudes h = 15 and ≃80% of neos with h = 25. hence, the large and small neos have different orbital distributions. the inferred flux of h < 18 bodies into the 3:1 resonance can be sustained only if the main-belt asteroids near the resonance drift toward the resonance at the maximal yarkovsky rate (≃2 × 10-4 au myr-1 for diameter d = 1 km and semimajor axis a = 2.5 au). this implies obliquities θ ≃ 0° for a < 2.5 au and θ ≃ 180° for a > 2.5 au, both in the immediate neighborhood of the resonance (the same applies to other resonances as well). we confirm the size-dependent disruption of asteroids near the sun found in previous studies. an interested researcher can use the publicly available neomod simulator to generate user-defined samples of neos from our model. | neomod: a new orbital distribution model for near-earth objects |
the asteroid (162173) ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. however, the collisional and orbital pathways from the original parent bodies to subkilometre rubble-pile asteroids are not yet well understood1-3. here we use hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous (c-type) asteroid ryugu4 are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. the bright boulders on ryugu can be classified into two spectral groups: most are featureless and similar to ryugu's average spectrum4,5, while others show distinct compositional signatures consistent with ordinary chondrites—a class of meteorites that originate from anhydrous silicate-rich asteroids6. the observed anhydrous silicate-like material is likely the result of collisional mixing between ryugu's parent body and one or multiple anhydrous silicate-rich asteroid(s) before and during ryugu's formation. in addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites7,8. they might sample different thermal-metamorphosed regions, which the returned sample will allow us to verify. hence, the bright boulders on ryugu provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids. | collisional history of ryugu's parent body from bright surface boulders |
the cretaceous-palaeogene mass extinction around 66 million years ago was triggered by the chicxulub asteroid impact on the present-day yucatán peninsula1,2. this event caused the highly selective extinction that eliminated about 76% of species3,4, including all non-avian dinosaurs, pterosaurs, ammonites, rudists and most marine reptiles. the timing of the impact and its aftermath have been studied mainly on millennial timescales, leaving the season of the impact unconstrained. here, by studying fishes that died on the day the mesozoic era ended, we demonstrate that the impact that caused the cretaceous-palaeogene mass extinction took place during boreal spring. osteohistology together with stable isotope records of exceptionally preserved perichondral and dermal bones in acipenseriform fishes from the tanis impact-induced seiche deposits5 reveal annual cyclicity across the final years of the cretaceous period. annual life cycles, including seasonal timing and duration of reproduction, feeding, hibernation and aestivation, vary strongly across latest cretaceous biotic clades. we postulate that the timing of the chicxulub impact in boreal spring and austral autumn was a major influence on selective biotic survival across the cretaceous-palaeogene boundary. | the mesozoic terminated in boreal spring |
context. the near-earth asteroid (3200) phaethon is an intriguing object: its perihelion is at only 0.14 au and is associated with the geminid meteor stream.aims: we aim to use all available disk-integrated optical data to derive a reliable convex shape model of phaethon. by interpreting the available space- and ground-based thermal infrared data and spitzer spectra using a thermophysical model, we also aim to further constrain its size, thermal inertia, and visible geometric albedo.methods: we applied the convex inversion method to the new optical data obtained by six instruments and to previous observations. the convex shape model was then used as input for the thermophysical modeling. we also studied the long-term stability of phaethon's orbit and spin axis with a numerical orbital and rotation-state integrator.results: we present a new convex shape model and rotational state of phaethon: a sidereal rotation period of 3.603958(2) h and ecliptic coordinates of the preferred pole orientation of (319°, -39°) with a 5° uncertainty. moreover, we derive its size (d = 5.1 ± 0.2 km), thermal inertia (γ = 600 ± 200 j m-2 s-1/2 k-1), geometric visible albedo (pv = 0.122 ± 0.008), and estimate the macroscopic surface roughness. we also find that the sun illumination at the perihelion passage during the past several thousand years is not connected to a specific area on the surface, which implies non-preferential heating. | near-earth asteroid (3200) phaethon: characterization of its orbit, spin state, and thermophysical parameters |
we present new constraints on the merging rates of planetary-mass and asteroid-mass primordial black hole binaries using limits on continuous waves (quasimonochromatic, quasi-infinite duration signals) derived from an all-sky search for isolated compact objects in the first six months of the third observing run (o3a) of ligo/virgo. we calculate the merging rates of these binaries in a model-independent way, and convert them to constraints on the primordial black hole abundance with minimal modeling assumptions. our results show that all-sky searches are sensitive to sources at most o (10 pc ) away for systems with chirp masses of o (10-5 m⊙) at gravitational-wave frequencies around 30 hz-40 hz. these results also show that continuous-wave searches could in the future directly probe the existence of planetary-mass and asteroid-mass primordial black holes, especially those in binaries with asymmetric mass ratios. furthermore, they demonstrate that new methods accounting for the full nonlinear gravitational-wave frequency evolution are needed to improve constraints on primordial black holes. | constraints on planetary and asteroid-mass primordial black holes from continuous gravitational-wave searches |
the fourth installment of the evolutionary system of mineralogy considers two stages of planetesimal mineralogy that occurred early in the history of the solar nebula, commencing by 4.566 ga and lasting for at least 5 million years: (1) primary igneous minerals derived from planetesimal melting and differentiation into core, mantle, and basaltic components and (2) impact mineralization resulting in shock-induced deformation, brecciation, melting, and high-pressure phase transformations.we tabulate 90 igneous differentiated asteroidal minerals, including the earliest known occurrences of minerals with ba, cl, cu, f, and v as essential elements, as well as the first appearances of numerous phosphates, quartz, zircon, and amphibole group minerals. we also record 40 minerals formed through high-pressure impact alteration, commencing with the period of asteroid accretion and differentiation. these stages of mineral evolution thus mark the first time that high pressures, both static and dynamic, played a significant role in mineral paragenesis. | an evolutionary system of mineralogy, part iv: planetesimal differentiation and impact mineralization (4566 to 4560 ma) |
the cool white dwarf sdss j124231.07+522626.6 exhibits photospheric absorption lines of eight distinct heavy elements in medium resolution optical spectra, notably including oxygen. the teff = 13 000 k atmosphere is helium-dominated, but the convection zone contains significant amounts of hydrogen and oxygen. the four most common rock-forming elements (o, mg, si, and fe) account for almost all the accreted mass, totalling at least 1.2 × 1024 g, similar to the mass of ceres. the time-averaged accretion rate is 2 × 1010 g s-1, one of the highest rates inferred among all known metal-polluted white dwarfs. we note a large oxygen excess, with respect to the most common metal oxides, suggesting that the white dwarf accreted planetary debris with a water content of ≈38 per cent by mass. this star, together with gd 61, gd 16, and gd 362, form a small group of outliers from the known population of evolved planetary systems accreting predominantly dry, rocky debris. this result strengthens the hypothesis that, integrated over the cooling ages of white dwarfs, accretion of water-rich debris from disrupted planetesimals may significantly contribute to the build-up of trace hydrogen observed in a large fraction of helium-dominated white dwarf atmospheres. | likely detection of water-rich asteroid debris in a metal-polluted white dwarf |
with the multiplication of social media platforms, which offer anonymity, easy access and online community formation, and online debate, the issue of hate speech detection and tracking becomes a growing challenge to society, individual, policy-makers and researchers. despite efforts for leveraging automatic techniques for automatic detection and monitoring, their performances are still far from satisfactory, which constantly calls for future research on the issue. this paper provides a systematic review of literature in this field, with a focus on natural language processing and deep learning technologies, highlighting the terminology, processing pipeline, core methods employed, with a focal point on deep learning architecture. from a methodological perspective, we adopt prisma guideline of systematic review of the last 10 years literature from acm digital library and google scholar. in the sequel, existing surveys, limitations, and future research directions are extensively discussed. | a systematic review of hate speech automatic detection using natural language processing |
the hayabusa2 spacecraft returned to earth from the asteroid 162173 ryugu on 6 december 2020. one day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. the container gas consists of helium and neon with an extraterrestrial3he/4he and20ne/22ne ratios, along with some contaminant terrestrial atmospheric gases. a mixture of solar and earth's atmospheric gas is the best explanation for the container gas composition. fragmentation of ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous he and the size distribution of the recovered ryugu grains. this is the first successful return of gas species from a near-earth asteroid.the hayabusa2 metal-sealed container successfully returned extraterrestrial he and ne as a gas phase from the asteroid ryugu. | first asteroid gas sample delivered by the hayabusa2 mission: a treasure box from ryugu |
we provide a quantitative understanding of raindrop impacts on sandy surfaces—a ubiquitous phenomenon relevant to many important natural, agricultural, and industrial processes. combining high-speed photography with high-precision laser profilometry, we investigate the dynamics of liquid-drop impacts on granular surfaces and monitor the morphology of resulting impact craters. remarkably, we discover a quantitative similarity between liquid-drop impacts and asteroid strikes in terms of both the energy scaling and the aspect ratio of their impact craters. such a similarity inspires us to apply the idea developed in planetary sciences to liquid-drop impact cratering, which leads to a model that quantitatively describes various features of liquid-drop imprints. | granular impact cratering by liquid drops: understanding raindrop imprints through an analogy to asteroid strikes |
as the target of the proposed asteroid impact & deflection assessment (aida) mission, the near-earth binary asteroid 65803 didymos represents a special class of binary asteroids, those whose primaries are at risk of rotational disruption. to gain a better understanding of these binary systems and to support the aida mission, this paper investigates the creep stability of the didymos primary by representing it as a cohesionless self-gravitating granular aggregate subject to rotational acceleration. to achieve this goal, a soft-sphere discrete element model (ssdem) capable of simulating granular systems in quasi-static states is implemented and a quasi-static spin-up procedure is carried out. we devise three critical spin limits for the simulated aggregates to indicate their critical states triggered by reshaping and surface shedding, internal structural deformation, and shear failure, respectively. the failure condition and mode, and shear strength of an aggregate can all be inferred from the three critical spin limits. the effects of arrangement and size distribution of constituent particles, bulk density, spin-up path, and interparticle friction are numerically explored. the results show that the shear strength of a spinning self-gravitating aggregate depends strongly on both its internal configuration and material parameters, while its failure mode and mechanism are mainly affected by its internal configuration. additionally, this study provides some constraints on the possible physical properties of the didymos primary based on observational data and proposes a plausible formation mechanism for this binary system. with a bulk density consistent with observational uncertainty and close to the maximum density allowed for the asteroid, the didymos primary in certain configurations can remain geo-statically stable without requiring cohesion. | creep stability of the proposed aida mission target 65803 didymos: i. discrete cohesionless granular physics model |
the formation channel of the tens of compact debris discs which orbit white dwarfs (wds) at a distance of 1 r⊙ remains unknown. asteroids that survive the giant branch stellar phases beyond a few au are assumed to be dynamically thrust towards the wd and tidally disrupted within its roche radius, generating extremely eccentric (e > 0.98) rings. here, we establish that wd radiation compresses and circularizes the orbits of supermicron to cm-sized ring constituents to entirely within the wd's roche radius. we derive a closed algebraic formula which well-approximates the shrinking time as a function of wd cooling age, the physical properties of the star and the physical and orbital properties of the ring particles. the shrinking time-scale increases with both particle size and cooling age, yielding age-dependent wd debris disc size distributions. | formation of planetary debris discs around white dwarfs - ii. shrinking extremely eccentric collisionless rings |
planetary systems such as our own are formed after a long process where matter condenses from diffuse clouds to stars, planets, asteroids, comets and residual dust, undergoing dramatic changes in physical and chemical state in less than a few million years. several studies have shown that the chemical composition during the early formation of a solar-type planetary system is a powerful diagnostic to track the history of the system itself. among the approximately 270 molecules so far detected in the ism, the so-called interstellar complex organic molecules (icoms) are of particular interest both because of their evolutionary diagnostic power and because they might be potential precursors of biomolecules, which are at the basis of terrestrial life. this chapter focuses on the evolution of organic molecules during the early stages of a solar-type planetary system, represented by the prestellar, class 0/i and protoplanetary disk phases, and compares them with what is observed presently in solar system comets. our twofold goal is to review the processes at the base of organic chemistry during solar-type star formation and, in addition, to possibly provide constraints on the early history of our own planetary system. | organic chemistry in the first phases of solar-type protostars |
we make use of a new hybrid method to simulate the long-term, multiple-orbit disc formation through tidal disruptions of rocky bodies by white dwarfs, at high-resolution and realistic semimajor axis. we perform the largest yet suite of simulations for dwarf and terrestrial planets, spanning four orders of magnitude in mass, various pericentre distances, and semimajor axes between 3 and 150 au. this large phase space of tidal disruption conditions has not been accessible through the use of previous codes. we analyse the statistical and structural properties of the emerging debris discs, as well as the ejected unbound debris contributing to the population of interstellar asteroids. unlike previous tidal disruption studies of small asteroids which form ring-like structures on the original orbit, we find that the tidal disruption of larger bodies usually forms dispersed structures of interlaced elliptic eccentric annuli on tighter orbits. we characterize the (typically power law) size distribution of the ejected interstellar bodies as well as their composition, rotation velocities, and ejection velocities. we find them to be sensitive to the depth (impact parameter) of the tidal disruption. finally, we briefly discuss possible implications of our results in explaining the peculiar variability of tabby's star, the origin of the transit events of ztf j0139+5245 and the formation of a planetary core around sdss j1228+1040. | tidal disruption of planetary bodies by white dwarfs - ii. debris disc structure and ejected interstellar asteroids |
we study the one-dimensional active ising model in which aligning particles undergo diffusion biased by the signs of their spins. the phase diagram obtained varying the density of particles, their hopping rate, and the temperature controlling the alignment shows a homogeneous disordered phase but no homogeneous ordered one, as well as two phases with localized dense structures. in the flocking phase, large ordered aggregates move ballistically and stochastically reverse their direction of motion. in what we termed the "aster" phase, dense immobile aggregates of opposite magnetization face each other, exchanging particles, without any net motion of the aggregates. using a combination of numerical simulations and mean-field theory, we study the evolution of the shapes of the flocks, the statistics of their reversal times, and their coarsening dynamics. solving exactly for the zero-temperature dynamics of an aster allows us to understand their coarsening, which shows extremal dynamics, while mean-field equations account for their shape. | flocking in one dimension: asters and reversals |
context. the hayabusa2 spacecraft launched by japan aerospace exploration agency has been conducting observations of the asteroid (162173) ryugu since june 2018. the telescopic optical navigation camera (onc-t) onboard hayabusa2 has obtained thousands of images under a variety of illumination and viewing conditions.aims: our objective is to examine and validate the camera calibration, derive a photometric correction for creating global albedo maps, and to interpret the photometric modeling results to characterize the surface of ryugu.methods: we observed (162173) ryugu with the gemini-south telescope, and combined these measurements with other published ground-based observations of the asteroid. the ground-based observations were compared with the data obtained by onc-t in order to validate the radiometric calibration mutually. we used a combination of the hapke disk-integrated and disk-resolved model equations to simultaneously analyze the combined ground- and spacecraft-based data.results: the average spectrum of ryugu was classified as cb-type following the smassii taxonomy and c/f-type following the tholen taxonomy based on spacecraft observations. we derived hapke model parameters for all seven color filters, which allowed us to photometrically correct images to within an error of <10% for ~80% of the image pixels used in the modeling effort. using this model, we derived a geometric albedo of 4.0 ± 0.5% (v band) for ryugu. the average reflectance factor at the standard illumination condition was 1.87 ± 0.14% in the v band. moreover we measured a phase reddening of (2.0 ± 0.7) × 10-3 μm-1 deg-1 for ryugu, similar to that observed for the asteroid (101955) bennu.conclusions: the global color map showed that the general trend was for darker regions to also be redder regions, however there were some distinct exceptions to this trend. for example, otohime saxum was bright and red while kibidango crater was dark and blue. the darkness and flatness of ryugu's reflectance might be caused by a high abundance of organic materials. | global photometric properties of (162173) ryugu |
asteroid crater retention ages have unknown accuracy because projectile-crater scaling laws are difficult to verify. at the same time, our knowledge of asteroid and crater size-frequency distributions has increased substantially over the past few decades. these advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. for d > 10 km diameter asteroids like ceres, vesta, lutetia, mathilde, ida, eros, and gaspra, the best matches occur when the ratio of crater to projectile sizes is f ∼ 10. the same scaling law applied to 0.3 < d < 2.5 km near-earth asteroids such as bennu, ryugu, itokawa, and toutatis yield intriguing yet perplexing results. when applied to the largest craters on these asteroids, we obtain crater retention ages of ∼1 billion years for bennu, ryugu, and itokawa and ∼2.5 billion years for toutatis. these ages agree with the estimated formation ages of their source families and could suggest that the near-earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. an alternative interpretation is that f ≫ 10, which would make their crater retention ages much younger. if true, crater scaling laws need to change in a substantial way between d > 10 km asteroids, where f ∼ 10, and 0.3 < d < 2.5 km asteroids, where f ≫ 10. | interpreting the cratering histories of bennu, ryugu, and other spacecraft-explored asteroids |
very recently, an extremely bright fast radio burst (frb) 200428 with two submillisecond pulses was discovered coming from the direction of the galactic magnetar sgr 1935+2154, and an x-ray burst (xrb) counterpart was detected simultaneously. these observations favor magnetar-based interior-driven models. in this letter, we propose a different model for frb 200428 associated with an xrb from sgr 1935+2154 in which a magnetar with high proper velocity encounters an asteroid of mass ∼1020 g. this infalling asteroid in the stellar gravitational field is first possibly disrupted tidally into a great number of fragments at a radius of ∼a few times 1010 cm, and then slowed around the alfvén radius by an ultra-strong magnetic field, and in the meantime two major fragments of mass ∼1017 g that cross magnetic field lines produce two pulses of frb 200428. the whole asteroid is eventually accreted onto the poles along magnetic field lines, impacting the stellar surface, creating a photon-e± pair fireball trapped initially in the stellar magnetosphere, and further leading to an xrb. we show that this gravitationally powered model can interpret all of the observed features self-consistently. | a magnetar-asteroid impact model for frb 200428 associated with an x-ray burst from sgr 1935+2154 |
although 25-50 per cent of white dwarfs (wds) display evidence for remnant planetary systems, their orbital architectures and overall sizes remain unknown. vibrant close-in (≃1 r⊙) circumstellar activity is detected at wds spanning many gyr in age, suggestive of planets further away. here we demonstrate how systems with 4 and 10 closely packed planets that remain stable and ordered on the main sequence can become unpacked when the star evolves into a wd and experience pervasive inward planetary incursions throughout wd cooling. our full-lifetime simulations run for the age of the universe and adopt main-sequence stellar masses of 1.5, 2.0 and 2.5 m⊙, which correspond to the mass range occupied by the progenitors of typical present-day wds. these results provide (i) a natural way to generate an ever-changing dynamical architecture in post-main-sequence planetary systems, (ii) an avenue for planets to achieve temporary close-in orbits that are potentially detectable by transit photometry and (iii) a dynamical explanation for how residual asteroids might pollute particularly old wds. | detectable close-in planets around white dwarfs through late unpacking |
we complete the survey for finite-source/point-lens (fspl) giant-source events in 2016--2019 kmtnet microlensing data. the 30 fspl events show a clear gap in einstein radius, $9 \mu{as} < \theta_{e} < 26 \mu{as}$, which is consistent with the gap in einstein timescales near $t_{e} \sim 0.5$ days found by mróz et al. (2017) in an independent sample of point-source/point-lens (pspl) events. we demonstrate that the two surveys are consistent. we estimate that the 4 events below this gap are due to a power-law distribution of free-floating planet candidates (ffps) $dn_{ffp}/d log m = (0.4 \pm 0.2)(m/38~m_{\oplus})^{-p}$/star, with $0.9 \lesssim p \lesssim 1.2$. there are substantially more ffps than known bound planets, implying that the bound planet power-law index $\gamma = 0.6$ is likely shaped by the ejection process at least as much as by formation. the mass density per decade of ffps in the solar neighborhood is of the same order as that of `oumuamua-like objects. in particular, if we assume that `oumuamua is part of the same process that ejected the ffps to very wide or unbound orbits, the power-law index is $p = 0.89 \pm 0.06$. if the solar system's endowment of neptune-mass objects in neptune-like orbits is typical, which is consistent with the results of \citet{wide-orbit}, then these could account for a substantial fraction of the ffps in the neptune-mass range. | free-floating planets, the einstein desert, and 'oumuamua |
we use a novel approach to model the entire cratering process resulting from impacts on small, weak asteroids that uses shock physics code calculations directly. we found that small-scale impacts (with a projectile size less than 1/150 of the target size) can significantly deform weak asteroids, causing global resurfacing at the same time. as a result, the collisional lifetime of the overall asteroid shapes is significantly lower than the traditionally used lifetime based on catastrophic disruption events. we also show that even very low asteroid cohesions can drastically influence the outcome of an impact. consequently, if the target is homogeneous and weaker than ≈10 pa, then nasa's double asteroid redirection test impact on dimorphos may not lead to a cratering event, as originally anticipated. rather, the impact may change the global morphology of the asteroid. our results, together with future observations by the esa's hera mission, will provide constraints regarding the evolution of the shapes and structures of small asteroids by subcatastrophic impacts. | global-scale reshaping and resurfacing of asteroids by small-scale impacts, with applications to the dart and hera missions |
exoplanet surveys have confirmed one of humanity's (and all teenagers') worst fears: we are weird. if our solar system were observed with present-day earth technology -- to put our system and exoplanets on the same footing -- jupiter is the only planet that would be detectable. the statistics of exo-jupiters indicate that the solar system is unusual at the ~1% level among sun-like stars (or ~0.1% among all stars). but why are we different? successful formation models for both the solar system and exoplanet systems rely on two key processes: orbital migration and dynamical instability. systems of close-in super-earths or sub-neptunes require substantial radial inward motion of solids either as drifting mm- to cm-sized pebbles or migrating earth-mass or larger planetary embryos. we argue that, regardless of their formation mode, the late evolution of super-earth systems involves migration into chains of mean motion resonances, generally followed by instability when the disk dissipates. this pattern is likely also ubiquitous in giant planet systems. we present three models for inner solar system formation -- the low-mass asteroid belt, grand tack, and early instability models -- each invoking a combination of migration and instability. we identify bifurcation points in planetary system formation. we present a series of events to explain why our solar system is so weird. jupiter's core must have formed fast enough to quench the growth of earth's building blocks by blocking the flux of inward-drifting pebbles. the large jupiter/saturn mass ratio is rare among giant exoplanets but may be required to maintain jupiter's wide orbit. the giant planets' instability must have been gentle, with no close encounters between jupiter and saturn, also unusual in the larger (exoplanet) context. our solar system system is thus the outcome of multiple unusual, but not unheard of, events. | solar system formation in the context of extrasolar planets |
we present results from applying the snad anomaly detection pipeline to the third public data release of the zwicky transient facility (ztf dr3). the pipeline is composed of three stages: feature extraction, search of outliers with machine learning algorithms, and anomaly identification with followup by human experts. our analysis concentrates in three ztf fields, comprising more than 2.25 million objects. a set of four automatic learning algorithms was used to identify 277 outliers, which were subsequently scrutinized by an expert. from these, 188 (68 per cent) were found to be bogus light curves - including effects from the image subtraction pipeline as well as overlapping between a star and a known asteroid, 66 (24 per cent) were previously reported sources whereas 23 (8 per cent) correspond to non-catalogued objects, with the two latter cases of potential scientific interest (e.g. one spectroscopically confirmed rs canum venaticorum star, four supernovae candidates, one red dwarf flare). moreover, using results from the expert analysis, we were able to identify a simple bi-dimensional relation that can be used to aid filtering potentially bogus light curves in future studies. we provide a complete list of objects with potential scientific application so they can be further scrutinised by the community. these results confirm the importance of combining automatic machine learning algorithms with domain knowledge in the construction of recommendation systems for astronomy. our code is publicly available.1 | anomaly detection in the zwicky transient facility dr3 |
this work investigates the macroscopic thermomechanical behavior of lunar boulders by modeling their response to diurnal thermal forcing. our results reveal a bimodal, spatiotemporally-complex stress response. during sunrise, stresses occur in the boulders' interiors that are associated with large-scale temperature gradients developed due to overnight cooling. during sunset, stresses occur at the boulders' exteriors due to the cooling and contraction of the surface. both kinds of stresses are on the order of 10 mpa in 1 m boulders and decrease for smaller diameters, suggesting that larger boulders break down more quickly. boulders ≤ 30 cm exhibit a weak response to thermal forcing, suggesting a threshold below which crack propagation may not occur. boulders of any size buried by regolith are shielded from thermal breakdown. as boulders increase in size (>1 m), stresses increase to several 10 s of mpa as the behavior of their surfaces approaches that of an infinite halfspace. as the thermal wave loses contact with the boulder interior, stresses become limited to the near-surface. this suggests that the survival time of a boulder is not only controlled by the amplitude of induced stress, but also by its diameter as compared to the diurnal skin depth. while stresses on the order of 10 mpa are enough to drive crack propagation in terrestrial environments, crack propagation rates in vacuum are not well constrained. we explore the relationship between boulder size, stress, and the direction of crack propagation, and discuss the implications for the relative breakdown rates and estimated lifetimes of boulders on airless body surfaces. | thermally induced stresses in boulders on airless body surfaces, and implications for rock breakdown |
weakly-supervised temporal action localization aims to localize action instances temporal boundary and identify the corresponding action category with only video-level labels. traditional methods mainly focus on foreground and background frames separation with only a single attention branch and class activation sequence. however, we argue that apart from the distinctive foreground and background frames there are plenty of semantically ambiguous action context frames. it does not make sense to group those context frames to the same background class since they are semantically related to a specific action category. consequently, it is challenging to suppress action context frames with only a single class activation sequence. to address this issue, in this paper, we propose an action-context modeling network termed acm-net, which integrates a three-branch attention module to measure the likelihood of each temporal point being action instance, context, or non-action background, simultaneously. then based on the obtained three-branch attention values, we construct three-branch class activation sequences to represent the action instances, contexts, and non-action backgrounds, individually. to evaluate the effectiveness of our acm-net, we conduct extensive experiments on two benchmark datasets, thumos-14 and activitynet-1.3. the experiments show that our method can outperform current state-of-the-art methods, and even achieve comparable performance with fully-supervised methods. code can be found at https://github.com/ispc-lab/acm-net | acm-net: action context modeling network for weakly-supervised temporal action localization |
we analyze the dynamics and observational signatures of axion clouds formed via the superradiant instability around primordial black holes, focusing on the mass range 1014 - 1018 kg where the latter may account for all the dark matter. we take into account the leading effects of axion self-interactions, showing that, even though these limit the number of axions produced within each cloud, a large number of superradiant axions become free of the black hole's gravitational potential and accumulate in the intergalactic medium or even in the host galaxy, depending on their escape velocity. this means that primordial black hole dark matter may lead to a sizeable astrophysical population of non-relativistic axions, with masses ranging from 0.1 ev to 1 mev, depending on the primordial black hole mass and spin. we then show that if such axions couple to photons their contribution to the galactic and extragalactic background flux, mainly in the x-ray and gamma-ray band of the spectrum, is already beyond current observational limits for a large range of parameters that are, therefore, excluded. we finish by showing the prospects of the athena x-ray telescope to further probe this co-existence of primordial black holes and axions. | superradiant axion clouds around asteroid-mass primordial black holes |
the shear and cohesive strengths of a rubble-pile asteroid could influence the critical spin at which the body fails and its subsequent evolution. we present results using a soft-sphere discrete element method to explore the mechanical properties and dynamical behaviors of self-gravitating rubble piles experiencing increasing rotational centrifugal forces. a comprehensive contact model incorporating translational and rotational friction and van der waals cohesive interactions is developed to simulate rubble-pile asteroids. it is observed that the critical spin depends strongly on both the frictional and cohesive forces between particles in contact; however, the failure behaviors only show dependence on the cohesive force. as cohesion increases, the deformation of the simulated body prior to disruption is diminished, the disruption process is more abrupt, and the component size of the fissioned material is increased. when the cohesive strength is high enough, the body can disaggregate into similar-size fragments, which could be a plausible mechanism to form asteroid pairs or active asteroids. the size distribution and velocity dispersion of the fragments in high-cohesion simulations show similarities to the disintegrating asteroid p/2013 r3, indicating that this asteroid may possess comparable cohesion in its structure and experience rotational fission in a similar manner. additionally, we propose a method for estimating a rubble pile’s friction angle and bulk cohesion from spin-up numerical experiments, which provides the opportunity for making quantitative comparisons with continuum theory. the results show that the present technique has great potential for predicting the behaviors and estimating the material strengths of cohesive rubble-pile asteroids. | rotational failure of rubble-pile bodies: influences of shear and cohesive strengths |
the yarkovsky effect is a thermal process acting upon the orbits of small celestial bodies, which can cause these orbits to slowly expand or contract with time. the effect is subtle ( $\left\langle {da}/{dt}\right\rangle \sim {10}^{-4}$ au my-1 for a 1 km diameter object) and is thus generally difficult to measure. we analyzed both optical and radar astrometry for 600 near-earth asteroids (neas) for the purpose of detecting and quantifying the yarkovsky effect. we present 247 neas with measured drift rates, which is the largest published set of yarkovsky detections. this large sample size provides an opportunity to examine the yarkovsky effect in a statistical manner. in particular, we describe two independent population-based tests that verify the measurement of yarkovsky orbital drift. first, we provide observational confirmation for the yarkovsky effect's theoretical size dependence of 1/d, where d is diameter. second, we find that the observed ratio of negative to positive drift rates in our sample is 2.34, which, accounting for bias and sampling uncertainty, implies an actual ratio of ${2.7}_{-0.7}^{+0.3}$ . this ratio has a vanishingly small probability of occurring due to chance or statistical noise. the observed ratio of retrograde to prograde rotators is two times lower than the ratio expected from numerical predictions from nea population studies and traditional assumptions about the sense of rotation of neas originating from various main belt escape routes. we also examine the efficiency with which solar energy is converted into orbital energy and find a median efficiency in our sample of 12%. we interpret this efficiency in terms of nea spin and thermal properties. | yarkovsky drift detections for 247 near-earth asteroids |
the chronology of dust formation in the early solar system remains controversial. chondrules are the most abundant high-temperature objects formed during the evolution of the circumsolar disk. considering chondrule formation, absolute lead‑lead (pbsbnd pb) ages and aluminum‑magnesium (26alsbnd 26mg) ages relative to calcium‑aluminum-rich inclusions (cais) provide inconsistent chronologies, with pbsbnd pb ages showing early and protracted chondrule formation episodes whereas 26alsbnd 26mg ages suggest that chondrule production was delayed by >1.5 ma. here, we develop a new method to precisely determine in situ26alsbnd 26mg ages of spinel-bearing chondrules, which are not affected by secondary asteroidal processes. our data demonstrate that 26alsbnd 26mg chondrule formation ages are actually 1 ma older than previously thought and extend over the entire lifetime of the disk. this shift in chondrule formation ages relative to cais, however, is not sufficient to reconcile the pbsbnd pb and 26alsbnd 26mg chronologies of chondrule and achondrite formation. thus, either chondrules' pbsbnd pb ages and volcanic achondrites' 26alsbnd 26mg ages are incorrect or the age of cais should be reevaluated at 4,568.7 ma to ensure consistency between chronometers. we favor the second hypothesis, given that (i) the canonical age of cais was determined using only 4 specimens and (ii) older ages of 4,568.2 ma have also been measured. we show that the adoption of 4,568.7 ma as the new canonical age of cais and the use of our new spinel-derived 26alsbnd 26mg ages enable reconciling the pbsbnd pb and 26alsbnd 26mg ages of chondrules and achondrites. this new chronology implies the existence of a 0.7-1 ma gap between the formation of refractory inclusions and chondrules, and supports the homogeneous distribution of 26al in the circumsolar disk. | a unified chronology of dust formation in the early solar system |
we modeled photometric observations of mutual events (eclipses and occultations) between the components of the binary near-earth asteroid (65803) didymos, the target of the double asteroid redirection test (dart) space mission, which were taken from 2003 to 2021. we derived parameters of the modified keplerian mutual orbit (allowing for a quadratic drift in the mean anomaly, which is presumably caused by an interplay between the byorp effect and mutual tides, or by differential yarkovsky force) of the secondary, called dimorphos, around the didymos primary and estimated its diameter. the j2000 ecliptic longitude and latitude of the orbital pole are 320.°6 ± 13.°7 and -78.°6 ± 1.°8, respectively, and the orbital period is 11.921624 ± 0.000018 hr at epoch jd 2,455,873.0 (asterocentric utc; all quoted uncertainties correspond to 3σ, except the density estimate below). we obtained the quadratic drift of the mean anomaly of 0.15 ± 0.14 deg yr-2. the orbital eccentricity is ≤0.03. we determined the ecliptic longitude and latitude of the radius vector of dimorphos with respect to didymos at the nominal time of the dart impact to dimorphos (jd 2,459,849.46875 geocentric utc) to be 222.°8 ± 7.°0 and -1.°6 ± 4.°2, respectively. we also estimated the bulk density of the system to be 2.37 ± 0.30 g cm-3 (1σ uncertainty). | preimpact mutual orbit of the dart target binary asteroid (65803) didymos derived from observations of mutual events in 2003-2021 |
using the s-band radar at arecibo observatory, we observed 16 psyche, the largest m-class asteroid in the main belt. we obtained 18 radar imaging and 6 continuous wave runs in november and december 2015, and combined these with 16 continuous wave runs from 2005 and 6 recent adaptive-optics (ao) images (drummond et al., 2016) to generate a three-dimensional shape model of psyche. our model is consistent with a previously published ao image (hanus et al., 2013) and three multi-chord occultations. our shape model has dimensions 279 × 232 × 189 km (± 10%), deff = 226 ± 23 km, and is 6% larger than, but within the uncertainties of, the most recently published size and shape model generated from the inversion of lightcurves (hanus et al., 2013). psyche is roughly ellipsoidal but displays a mass-deficit over a region spanning 90° of longitude. there is also evidence for two ∼50-70 km wide depressions near its south pole. our size and published masses lead to an overall bulk density estimate of 4500 ± 1400 kgm-3. psyche's mean radar albedo of 0.37 ± 0.09 is consistent with a near-surface regolith composed largely of iron-nickel and ∼40% porosity. its radar reflectivity varies by a factor of 1.6 as the asteroid rotates, suggesting global variations in metal abundance or bulk density in the near surface. the variations in radar albedo appear to correlate with large and small-scale shape features. our size and psyche's published absolute magnitude lead to an optical albedo of pv = 0.15 ± 0.03, and there is evidence for albedo variegations that correlate with shape features. | radar observations and shape model of asteroid 16 psyche |
the secular approximation of the hierarchical three body systems has been proven to be very useful in addressing many astrophysical systems, from planets to stars to black holes. in such a system, two objects are on a tight orbit and the tertiary is on a much wider orbit. here, we study the dynamics of a system by taking the tertiary mass to zero and solve the hierarchical three body system up to the octupole level of approximation. we find a rich dynamics that the outer orbit undergoes due to gravitational perturbations from the inner binary. the nominal result of the precession of the nodes is mostly limited for the lowest order of approximation; however, when the octupole level of approximation is introduced, the system becomes chaotic, as expected, and the tertiary oscillates below and above 90°, similarly to the non-test particle flip behavior. we provide the hamiltonian of the system and investigate the dynamics of the system from the quadrupole to the octupole level of approximations. we also analyze the chaotic and quasi-periodic orbital evolution by studying the surfaces of sections. furthermore, including general relativity, we showcase the long-term evolution of individual debris disk particles under the influence of a far-away interior eccentric planet. we show that this dynamics can naturally result in retrograde objects and a puffy disk after a long timescale evolution (a few gyr) for initially aligned configuration. | the eccentric kozai-lidov mechanism for outer test particle |
nasa's double asteroid redirection test (dart) spacecraft is planned to impact the natural satellite of (65803) didymos, dimorphos, at around 23:14 utc on 2022 september 26, causing a reduction in its orbital period that will be measurable with ground-based observations. this test of kinetic impactor technology will provide the first estimate of the momentum transfer enhancement factor β at a realistic scale, wherein the ejecta from the impact provide an additional deflection to the target. earth-based observations, the liciacube spacecraft (to be detached from dart prior to impact), and esa's follow-up hera mission, to launch in 2024, will provide additional characterizations of the deflection test. together, hera and dart comprise the asteroid impact and deflection assessment cooperation between nasa and esa. here, the predicted dynamical states of the binary system upon arrival and after impact are presented. the assumed dynamically relaxed state of the system will be excited by the impact, leading to an increase in eccentricity and a slight tilt of the orbit, together with enhanced libration of dimorphos, with the amplitude dependent on the currently poorly known target shape. free rotation around the moon's long axis may also be triggered, and the orbital period will experience variations from seconds to minutes over timescales of days to months. shape change of either body, due to cratering or mass wasting triggered by crater formation and ejecta, may affect β, but can be constrained through additional measurements. both byorp and gravity tides may cause measurable orbital changes on the timescale of hera's rendezvous. | predictions for the dynamical states of the didymos system before and after the planned dart impact |
the asteroid impact & deflection assessment (aida) mission is a joint cooperation between european and us space agencies that consists of two separate and independent spacecraft that will be launched to a binary asteroid system, the near-earth asteroid didymos, to test the kinetic impactor technique to deflect an asteroid. the european asteroid impact mission (aim) is set to rendezvous with the asteroid system to fully characterize the smaller of the two binary components a few months prior to the impact by the us double asteroid redirection test (dart) spacecraft. aim is a unique mission as it will be the first time that a spacecraft will investigate the surface, subsurface, and internal properties of a small binary near-earth asteroid. in addition it will perform various important technology demonstrations that can serve other space missions. the knowledge obtained by this mission will have great implications for our understanding of the history of the solar system. having direct information on the surface and internal properties of small asteroids will allow us to understand how the various processes they undergo work and transform these small bodies as well as, for this particular case, how a binary system forms. making these measurements from up close and comparing them with ground-based data from telescopes will also allow us to calibrate remote observations and improve our data interpretation of other systems. with dart, thanks to the characterization of the target by aim, the mission will be the first fully documented impact experiment at asteroid scale, which will include the characterization of the target's properties and the outcome of the impact. aida will thus offer a great opportunity to test and refine our understanding and models at the actual scale of an asteroid, and to check whether the current extrapolations of material strength from laboratory-scale targets to the scale of aida's target are valid. moreover, it will offer a first check of the validity of the kinetic impactor concept to deflect a small body and lead to improved efficiency for future kinetic impactor designs. this paper focuses on the science return of aim, the current knowledge of its target from ground-based observations, and the instrumentation planned to get the necessary data. | science case for the asteroid impact mission (aim): a component of the asteroid impact & deflection assessment (aida) mission |
the early impact bombardment of mars has been linked to the bombardment history of the inner solar system as a whole. however, the timing and number of basin-forming impacts on mars are poorly constrained. the borealis basin--thought to be the largest and oldest known martian impact basin--forms the crustal dichotomy between the northern lowlands and southern highlands. four unambiguous large basins post-dating borealis have been identified, but as many as 32 additional basins larger than 1,000 km in diameter have been proposed. here we use gravity and topography analyses to show that the crustal dichotomy boundary was excavated by only one later impact basin (isidis), which probabilistically indicates that fewer than 12 large basins across the globe could post-date the boundary and pre-date the established younger basins. moreover, the relatively pristine topography and crustal thickness at the dichotomy boundary indicates that younger basins should be similarly well preserved. this suggests that the post-borealis large basin population is limited to only the four known younger basins, with estimated ages between 3.8 and 4.1 gyr ago (ga). we present geochemical arguments that borealis dates to near 4.5 ga. combined with monte carlo simulations, we argue that, instead of a gradually declining impactor flux, a lull in large basin-forming impacts occurred between about 4.1 and 4.4 ga on mars, separating the endgame of accretion from a putative late heavy bombardment similar to that proposed for the moon and asteroid belt. | a post-accretionary lull in large impacts on early mars |
the ablation of cosmic dust injects a range of metals into planetary upper atmospheres. in addition, dust particles which survive atmospheric entry can be an important source of organic material at a planetary surface. in this study the contribution of metals and organics from three cosmic dust sources - jupiter-family comets (jfcs), the asteroid belt (ast), and halley-type comets (htcs) - to the atmospheres of earth, mars and venus is estimated by combining a chemical ablation model (cabmod) with a zodiacal cloud model (zody). zody provides the mass, velocity, and radiant distributions for jfc, ast, and htc particles. jfcs are shown to be the main mass contributor in all three atmospheres (68% for venus, 70% earth, and 52% for mars), providing a total input mass for venus, earth and mars of 31 ± 18 t d-1, 28 ± 16 t d-1 and 2 ± 1 t d-1, respectively. the mass contribution of ast particles increases with heliocentric distance (6% for venus, 9% for earth, and 14% for mars). a novel multiphase treatment in cabmod, tested experimentally in a meteoric ablation simulator, is implemented to quantify atmospheric ablation from both the silicate melt and fe-ni metal domains. the ratio of fe:ni ablation fluxes at earth, mars and venus are predicted to be close to their ci chondritic ratio of 18, in agreement with mass spectrometric measurements of fe+:ni+ = 20-8+13 in the terrestrial ionosphere. in contrast, lidar measurements of the neutral atoms at earth indicate fe:ni = 38 ± 11, and observations by the neutral gas and ion mass spectrometer on the maven spacecraft at mars indicate fe+:ni+ = 43-10+13. given the slower average entry velocity of cosmic dust particles at mars, the accretion rate of unmelted particles in mars represents 60% of the total input mass, of which a significant fraction of the total unmelted mass (22%) does not reach an organic pyrolysis temperature (~900 k), leading to a flux of intact carbon of 14 kg d-1. this is significantly smaller than previous estimates. | cosmic dust fluxes in the atmospheres of earth, mars, and venus |
analyses of meteorites and theoretical models indicate that some carbonaceous near-earth asteroids may have been thermally altered due to radiative heating during close approaches to the sun1-3. however, the lack of direct measurements on the subsurface doesn't allow us to distinguish thermal alteration due to radiative heating from parent-body processes. in april 2019, the hayabusa2 mission successfully completed an artificial impact experiment on the carbonaceous near-earth asteroid (162173) ryugu4,5, which provided an opportunity to investigate exposed subsurface material and test potential effects of radiative heating. here we report observations of ryugu's subsurface material by the near-infrared spectrometer (nirs3) on the hayabusa2 spacecraft. reflectance spectra of excavated material exhibit a hydroxyl (oh) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. the strength and shape of the oh feature suggests that the subsurface material experienced heating above 300 °c, similar to the surface. in contrast, thermophysical modelling indicates that radiative heating cannot increase the temperature above 200 °c at the estimated excavation depth of 1 m, even at the smallest heliocentric distance possible for ryugu. this supports the hypothesis that primary thermal alteration occurred on ryugu's parent body. | thermally altered subsurface material of asteroid (162173) ryugu |
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